Peptide YY (PYY) enhances feeding and drinking more potently than does neuropeptide Y after central administration. Chronic administration of PYY every 6 h for 48 h causes massive food ingestion. Tolerance to this effect of PYY does not appear to develop. This data suggests that PYY is one of the most potent orexigenic substances yet to be identified. PYY may play a role in the pathogenesis of bulimic syndromes.