Cholecystokinin (CCK) inhibits food intake in adults. This paper describes research examining the ability of CCK to affect feeding in infant rats and the role of CCK in the developmentally emerging ability of the rat pup to inhibit ingestion in response to sensory characteristics of food. First, data will be described from studies that asked if the CCK system is functional in preweanling rats. Specifically, these studies examined whether exogenous and endogenous CCK can decrease intake of the infant rat during independent ingestion (of a milk diet, away from the dam). In addition, the ability of exogenous CCK to activate central feeding-control areas in the brain stem and hypothalamus in infant rats was examined by C-FOS staining. Next, experiments examining which specific intake-inhibitory sensory aspects of food are mediated by CCK will be described. The volume, hypertonicity, fat, carbohydrate and protein content of a preload were separately manipulated in different studies, followed closely by a 30-min test meal. The selective CCK(1) receptor antagonist devazepide was used to assess CCK mediation of the control of intake produced by particular sensory aspects of food, at the earliest age in which this ability to control intake appears. Finally, the pattern of independent ingestion in infant OLETF rats lacking CCK(1) receptors was examined. The results suggest that the CCK intake-inhibitory mechanism is potentially available to the young, suckling pup even before it starts to feed on its own. However, it appears to mediate only a portion of the controls of intake during nursing and early stages of weaning. Some aspects of the CCK system (e.g., forebrain-hindbrain connections) and CCK's role in mediating the effects of other stimulus aspects of food apparently undergo a post-weaning maturational process.