The discovery of opiate receptors and endogenous opioid peptides within the central nervous system has resulted in a number of speculations concerning the physiological significance of these peptides. In the present article, we review the evidence suggesting a primary role for some of the opioid peptides as regulators of ingestive behavior. In particular, we elaborate a hypothesis in which we suggest that in some species opioid peptides may play a role as a tonic inducer of ingestive behaviors, held in check by a variety of neuropeptides and monoamines. This review explores in detail the role of the opioid peptides as major mediators of the reward system and as a link between reward and feeding behaviors. Finally, a teleological role for opioid peptides in species preservation, which may explain the discrepancies in the role of the opioid peptides in feeding behavior in different species is proposed. It is suggested that the feeding profile of the animal provides important clues as to whether or not the animal has an opiate-sensitive feeding system. We stress that interactions with ingested nutrients and the milieu interieur provide an important means by which animals modulate the opiate-entrained feeding drives.