In Mahābhārata, several times iṣṭāpūrta or iṣṭāpūrta phala has been mentioned. So is in Rāmāyaṇa. But wherever this term has been used, it is with a sense of disappointment. It is said in the following sense– ‘If you commit this offense, you will not have the virtuous fruit of your iṣṭāpūrta.

In Rāmāyaṇa, when Viśvāmitra came to Daśaratha, to seek relief from the attacks of Tāḍakā and Mārīca-Suvāhu, Daśaratha at first promised that he would fulfill all the desired of Viśvāmitra, but when Viśvāmitra wanted to take Rāma, Daśaratha was not willing to let the young Rāma to go with him. Then Viśvāmitra was very annoyed, said several offensive words to Daśaratha, and finally, Vaśiṣṭha said to him, if you don̍t keep your promise, all your fruits of Iṣṭāpūrta will be destroyed–
iṣṭāpūrtavadho bhūyāttasmād rāmaṃ visarjaya.

In the Gālava legend in Mahābhārata, there is a similar śloka for that. Here, guru Viśvāmitra did not demand dakṣiṇā from disciple Gālava, but Gālava insisted on giving dakṣiṇā. Viśvāmitra, then, getting irritated, demanded something which was beyond the capacity of the disciple. Now, Gālava, unable to find any wayout, started lamenting, and says– if someone promises to do something and then cannot execute it, acquires the sin of telling a lie, and all his virtues acqired as iṣṭāpūrta get lost–
pratiśruti kariṣyeti kartavyaṃ tadakurvataḥ
mithyāvacanadagdhasya iṣṭāpūrtaṃ praṇaśyati.

From these two examples in Rāmāyaṇa and Mahābhārata, it appears that iṣṭāpūrta is such a thing, by performing which act, one acquires virtue, and nobody wishes to lose. So comes the question, what is the significance of iṣṭāpūrta? In Veda, the term iṣṭāpūrta has been used only once, and it is related to pitṛloka. Here, one addresses one̍s own father, saying– “In that wonderful svarga, you meet you forefathers, and meet Yama, and get related to your so-long acquired iṣṭāpūrta–
saṃgacchasva pitṛbhiḥ sa yameneṣṭāpūrtena parame vyoman.

Here, explaining the meaning of iṣṭāpūrta, Sāyanācārya has written– iṣṭāpūrta is the fruit of śrauta-smārta dāna–
śrauta-smārta dānaphalena saṃgcchsva.

The English translation of this is clearer– “The cumulative spiritual result or merit due to a man̍s performance of sacrifices and charitable acts.”

Later, iṣṭāpūrta has entered into the the performable duties prescribed in Dharmaśāstra, but in the Vedic period itself, it had been incorporated as part of yajñkarma, and its meaning is quite clear. In one statement in Atharvaveda, it has been made clear that the yajñkarma(s) or beneficial deeds performed by the forefathers, yield their merit upon the future generations as well, and protect their wealth, fame etc. The mantra says– let the iṣṭāpūrta of our forefathers protect us–
iṣṭāpūrtamavatu naḥ pitṛṇām

Iṣṭāpūrta also determines the stability and happiness the heavenly abode, for those who, after the end of their mortal life, go away to the path of heaven. For attaining this absolute joy, the ākūta mantra is chanted. In that ākūtamantra of Yajurveda, it is said that– let the fruits of his iṣṭāpūrta come out as prominent, when he comes to the path of the divine vehicle. Whatever yajña he has performed in life, whatever donations made and dakṣiṇa paid– let Agni speak of all those good deeds in front of the gods, and let him be placed among the gods.

In the Vedic tradition, īṣṭāpūrta has been recognised as dharma, in Mahābhārata, Purāṇa and the Dharmaśāstra(s). The commentator of such a famous text as Yājñavalkya-smṛti has cited two śloka(s) of Mahābhārata, and said, indicating at a definition of iṣṭāpūrta– whatever is offered as āhūti to gārhapatya agni, or the homa performed in gārhapatya, āhavanīya and dakṣiṇāgni, or whatever is put inside the yajña-altar as āhūti– is called ̍iṣṭa̍ according to its Vedic terminology. And beneficial acts like digging a pond, well or such waterbodies, establishing temples, giving food, making orchards– for the sake of the public– these are called pūrtakarma–
ekāgnikarma havanaṃ tretāyāṃ yaccha hūyate
antarvedyāṃ ca yaddānamiṣṭamityabhidhīyate.
vāpīkūpataḍāgāni devatāyatanāni ca.
annapradānamārāmaḥ pūrtamityabhidhīyate.

These two śloka(s), as cited in Aparārka-ṭīkā, could not be found in the popular editions of Mahābhārata mostly in circulation. But we may assume that, since a great scholar like Aparārka has cited these two as śloka(s) from Mahābhārata, they must be included in some indegenous edition of Mahābhārata. Even if these are not to be found in Mahābhārata, but if we look at the important texts related to Dharmaśāstra(s), it can be understood that an attempt had been going on to find a definition of iṣṭāpūrta. Raghunandana, the next great exponent of Smṛti-tradition, in his text called Malamāsatatva, citing a predecessor-Ācārya called Jātūkarṇa– also says that daily agnihotra, tapasyā, satya (truth) , performing the acts prescribed in Veda(s), serving the guests and Vaiśvadeva karma (act dedicated to the Viśvadeva)– these are iṣṭa. And the acts of digging ponds, waterbodies etc.– are pūrta karma. The śloka attributed to Jātūkarṇa is just a repetition of the Mahābhārata-śloka, as cited by Aparārka. Here it is also said that all the śrauta-smārta karma(s) can be performed by the three jāti(s), Brāhmaṇa, Kṣatriya and Vaiśya (those who are known as dvija, ̍twice born̍). Śūdra(s) are not entitled to the ̍iṣṭa̍ karma(s), but they can perform pūrtakarma . It is to be noted that, the right of women to perform pūrtakarma has also been recognised– evaṃ strīṇāmāpi pūrtādhikāraḥ. Raghunandana has given an evidence from Vṛhaspati- smṛti–
śūdrasya pūrtādhikāramāha jātūkarṇaḥ.
— vāpīkūpataḍāgādi-devatāyatanni ca
annpradānamārāmāḥ pūrtamityabhidhiyate.
ārāmaḥ puṣpaphalopayaheturbhūbhāgaḥ
̍agnihotraṃ tapaḥ satyaṃ vedānāñcanupālanaṃ.
atithyaṃ vaiśvadevaśca iṣṭamityabhidhīyate.
iṣṭāpūrtaṃ dvijātīnāṃ dharmaḥ sāmānya ucyate.
adhikārī bhavecchudraḥ purte dharme na vaidike.̍
vaidike adhyanasādhye
agnihotrādaviti ratnākaraḥ.
evaṃ strīṇāmāpi pūrtādhikāraḥ.

It is to be noted that, in the Vedic age iṣṭāpūrta-karma was only entitled to the Brāhmaṇa(s). Even Kṣatriya(s) did not have right to perform these, nothing to say about the Vaiśya(s). Perhaps that is why it is said in an ancient Brāhmaṇ-text like Taittirīya Brāhmaṇa–
iṣṭāpūrtaṃ brāhmaṇsya.

But society and the requirements of social life is such a continuous process, that it has been understood that — apart from agnihortra and other activities related to yajña, poor and ascetic brāhmaṇa(s) could not afford to perform such acts as digging ponds and waterbodies, for the benefit of the general public. So Kṣtriya(s) came to to entitled to the Kṣatriya(s); and since Kṣatriya(s) also performed yāga-yajña(s) quite well, it did not take long for them to be entitled to the right of performing ̍iṣṭa̍. This statement is supported in Aitareya Brāhmaṇa.

In Aitareya Brāhmaṇa, there is a performance so that iṣṭāpūrta become undeclining (iṣṭpūrta aparijjyāni). In the beginning of this ceremony, it is said– in ancient times, seeing Rājanya, Vaiśya and Śūdra, Yajña (personified) fled away. Both Brāhmaṇa and Kṣatriya chased him. Brahmaṇa had his weapons in his hand, and Kṣatriya had bow and arrows. When Yajña was running away, Brāhmaṇa ran after him and finally caught him, but the Kṣatriya could not, and so he retreated. After being caught, Yajña found his own wepaons (whatever equipments were required for performing yajña)– such as sphya, kapāla, sūrpa, kṛṣṇājina — were exactly same as the weapons of Brāhmaṇa, he came under the authority of Brāhmaṇa. Now the Kṣatriya came, and asked the Brahmaṇa to invite him to the yajña as well. Brāhmaṇ said, “You may come, but you have to leave your weapons, and come dressed as a Brāhmaṇa, with the yajña-equipments of a Brāhmaṇa. Since then, while performing yajña, Kṣatriya(s) come assuming the appearance of Brāhmaṇa.

After stating this legend, Aitareya Brāhmaṇ says–

Now, after the Kṣatriya yajamāna̍s right to perform had been established, there had been a customary offering to the Āhavanīya agni, before the yajñadīkṣā (ritual initiation for the yajña-karma) so that the iṣṭāpūrta of the yajamāna remains undeclined–
athāta iṣṭāpūrtasyāparijyāniḥ kṣatriyasya
yajñamānasya, sa purastād dīkṣāyāḥ…

From this description we may understand that the right of kṣatriya(s) to iṣṭāpūrta karma came later, and the rights of Vaiśya(s) came much later. Since Brāhmaṇa(s) had practical difficulty in performing pūrtakarma, gradually the right of women and all the four jāti(s)– who could afford it– in such acts also came to be recognised due to social reasons. This came in course of time, and it becomes clear when Sāyanācārya defines iṣṭa and pūrta. Citing the predecessor-scholars, he has mentioned that the dharma of iṣṭāpūrta has been created through the gradual evolution of varṇāśrama. Acts like digging the well, pond etc. can be done by everybody. Finally he says that– in the oinion of many others, iṣṭa is śrauta karma prescribed by Veda and pūrta is Smārta karma, that is, prescribed in Smṛti–
varṇāśramāṇvyī dharma iṣṭaṃ pūrtamathetarat
prapātaṭākādirūpaṃ tañca sarvatra dṛṣyate.
smārtaṃ pūrtaṃ śrautamiṣṭamiti kecidihocire.

In Vedic rituals, iṣṭāpūrta has been given a great significance, but it is not sung in the same terms of glory, in the Upaniṣad(s) which are mostly singing the glory of mukti and mokṣa through renouncement. Puṇya also creates mortal binding for the living creatures, the cycle of life and death continues in this way. In Muṇḍakopaniṣad, it has been said without any doubt , those who think of iṣṭāpūrta as the greatest of all Puruṣārtha(s), are not capable of thinking of anything more than this, they cannot think of self-knowledge (ātmajñāna). The heavenly abode they attain to, is as a result of their good deeds. But after that, they have to be reborn in this world–

iṣṭāpūrtāṃ manyamānā variṣṭhaṃ
nānyacchreyo vedayante pramūḍāḥ
nākasya pṛṣṭhe te sukṛte̍nubhutvemaṃ
lokaṃ hīnataraṃ vā viśanti

This crisis of people who seek salvation is also described in Chāndogya Upaniṣad and Prasnopaniṣad.

For the reclusive minded people, who seek to renounce the world, there is little significance of iṣṭāpūrta karma. However, for common householders and wealthy people, it is certainly of great importance, because it is the cause of prosperity in this earthly life and beyond.

Especially in the first part of iṣṭāpūrta, that is, iṣṭa, consists of yajña, dāna , tapasyā and other rituals, where Brahminical practices assume the most important significance; and pūrtakarma is ̍lower̍ in comparison to that, and thus discussed in Purāṇa and Smṛti(s). After a certain period,  it is found that one or two Purāṇa(s) have come out of the Brahminnical garb and state that– by iṣṭakarma the heaven may be attained. But pūrtakarma, which can be performed by people belonging to all the varṇa(s), is the way to mokṣa–

iṣṭāpūrtaṃ dvijātīnāṃ prathamaṃ dharmasādhanam

iṣṭena labhate svargaṃ pūrte mokṣañca vindati.

This statement in Varāhapurāṇa introduces an important social indication. Whereas in other Purāṇa(s), there is no right  entitled to  women and Śūdra(s)– and here it is said that women and  Śūdra(s) can also attain liberation though such civil works of welfare– is a sign of liberalism of the Purāṇa.  In this part of Varāhapurāṇa, it is further said that planting a number of saplings is very fruitful to attain mokṣa– which is a rather modern concept, and related to ecology. Especially such Pūrtakarma(s) as digging ponds, as in the Anuśāsanaparva of Mahābhārta, and establishing monasteries, cirles etc., as in Padmapurāṇa, bears the indication of attaining mokṣa.