Early Vedic ritual was centred upon sacrifice (yajna) performed on plots of land temporarily consecrated for the purpose (vedi), not the worship of images (murti) in permanent temples (mandirs). The purpose of sacrifice seems to have been to ensure that the gods and the natural phenomena associated with them such as rain (Indra) or the sun (Surya) would benefit the persons making the sacrifice in this life, as well as ensuring a happy afterlife, which in ancient times seems to have been envisaged as the world of the ancestors rather than reincarnation and liberation. The importance of sacrifice is reflected in the role accorded to it in Vedic creation myth where the gods perform the primordial and prototypical act of sacrifice when they dismember Primal Man from whom both the physical universe and human society originate (see The cosmos and the natural world).
The centrality of sacrifice in ancient times is demonstrated by the membership of the Vedic pantheon in which important gods such as Agni (Fire), the goddess Vac (Speech) and Soma (an intoxicant) were divine forms of parts of the sacrifice. Soma, for example, was an intoxicant extracted from a plant source, apparently used in ancient Zoroastrian as well as Vedic rituals. It was both drunk and offered to the gods during some sacrifices. Exactly which plant or mixture of plants it was and what its effects were are debated, some arguing that it was hallucinogenic and suggesting psilocybin or other mushrooms, others that it was more like amphetamine in effects, and suggesting the plant ephedra, others a mixture of extracts of poppy, cannabis and an ephedra-like plant, still others suggesting various other plants, perhaps something with similar effects to the South American ayahuasca. Why its use was discontinued is not known. It may be that the plant or plants became hard to find or the knowledge of the process lost. Some Hindus argue that it was not a drug but a metaphor for intoxication with divine love.
Some sacrifices were fairly simple such as the Agnihotra (Oblation to Agni), which was an obligation for the twice-born (dvija) householder who should make twice daily offerings of milk and water into the household ritual fires while reverencing the gods, the ancestors, and the seven rishis (sages/seers). Other sacrifices required the expertise of specialist priests, and could involve animal offerings as well as offerings of grains, fruit, ghee (clarified butter) and milk. Among the most elaborate and expensive sacrifices was the Ashvamedha (horse sacrifice), a ritual of sovereignty limited to royalty. Although not widespread today, some animal sacrifice continues to be a feature of Hinduism in parts of India and Nepal, especially in worship of the Goddess such as the goat offerings to Kali in her Kolkata temple and animal sacrifices to other goddesses such as Shitala the goddess of smallpox and other infectious diseases.
As the Vedic period developed, the sacrifice gained in importance so that the performance of the sacrifice in itself was believed to be essential to the maintenance of the cosmos. The pantheon was also changing with deities of a more abstract or universal character (though often maintaining links with creation and the sacrifice) such as Hiranyagabha (the cause of the universe), Brihaspati (the divine priest) and Prajapati (the source of creation and the power of sacrifice). Indeed, by the close of the Vedic period, ideas about sacrifice differed significantly from earlier Vedic ones both in respect of nature and purpose. The power of ritual came to be understood as a mental process internal to the individual rather than occurring literally and externally. Hence, in the Shvetasvatara Upanishad, the sacrifice becomes a metaphor in which the sacrificial kindling of fire is reinterpreted in terms of the body, mantra and meditation. Likewise, the sacrifice was seen in terms of the predominantly otherworldly goals associated with the stress upon moksha (release, liberation) and the growing importance of renunciation in comparison to the more material concerns of the householder. This is evident in the Chandogya Upanishad where sacrifice is identified with brahmacharya (the celibate state of traditional Vedic studentship) as the means to self-realisation.
Elements of Vedic ritual continue to this day, especially the importance of fire and fire offerings in lifecycle ceremonies such as marriage. Versions of the Agnihotra continue to be performed to ensure the welfare of the household and to contribute towards order and harmony on a broader natural and social basis. Vedic deities and stories connected with them (such as Indra the storm god) still feature in Hindu tradition, but in general much Vedic ritual as well as some of the mythology and deities with which it was associated has declined in importance and been replaced over the millennia by other deities becoming more important and by devotional practices such as image worship. However, Vedic ritual focused on the sacred fire has been revived in the modern era, in part because of the campaigns of the nineteenth century Arya Samaj which reintroduced Agnihotra and other fire-based rituals to replace image worship.