The ontogenetic development of postingestive inhibitory control of ingestion by an oil preload was examined in preweaning rats. Gastric preloads (5% b.wt) of water, mineral oil or corn oil were administered 5 min before a 30-min intake test in which pups licked milk from the floor of a test chamber. Preloads of corn oil decreased intake significantly compared with preloads of mineral oil or water on Postnatal Days 15 and 18, but not on Postnatal Day 12. Because preloads of corn oil reduced intake ore than preloads of mineral oil, it is possible that the inhibitory effect of corn oil is due to its fats rather than to its oily texture. Cholecystokinin (CCK), presumably released from the small intestine, apparently mediates part of this inhibitory effect because pretreatment with devazepide, a specific CKKA antagonist, significantly reduced the inhibition produced by corn oil.