The ability of the octapeptide of cholecystokinin (CCK-8) to inhibit independent ingestion was examined in 8-h deprived 1- to 10-day-old rat pups consuming milk from the surface of a test chamber. CCK significantly reduced the volume of milk ingested in 1-day-old rat pups at a dose of 100 ng/kg. The threshold effective dosage for inhibition of intake increased with age up to 10 days of age. The specificity of this suppression was evaluated in three ways. First, behavioral observation indicated that ingestive but not noningestive behaviors were suppressed by CCK. Second, desulfated CCK-8 was ineffective in suppressing milk ingestion. Finally, CCK inhibited milk intake stimulated by deprivation but did not affect dehydration-induced milk ingestion in 3-day-old rat pups. In addition to inhibiting intake, CCK inhibited the rate of gastric emptying of saline test meals in 1- and 3-day-old rat pups, and the threshold dosages for this suppression were identical to the thresholds for the inhibition of ingestion. These results indicate that the gastrointestinal peptide CCK is both behaviorally and physiologically active in newborn rat pups and support a role for gastric inhibition in the satiety action of CCK.