It has been reported that systemic injections of cholecystokinin (CCK) elicit the behavioral characteristics of satiety, including sleep, in rats. CCK is a potent stimulator of insulin secretion, and insulin is hypothesized to be involved in sleep and feeding regulation. The purpose of the current experiments was to study the possible role of endogenous insulin in the food-intake-reducing and hypnogenic effects of intraperitoneally (IP) administered CCK. Normal and streptozotocin (STR)-diabetic rats were injected with isotonic saline or CCK (10 and 50 micrograms/kg) at dark onset, and sleep-wake activity was determined for the next 12 h. There were no significant differences between the baseline sleep-wake activity of normal and diabetic rats. IP injection of CCK elicited a selective increase in nonrapid-eye-movement sleep in both groups during the first postinjection hour. In a separate experiment, the effects of CCK (10 micrograms/kg) on food intake were determined in control and diabetic rats; CCK suppressed the 1-h food intake in both groups. In a third experiment, the effects of CCK treatment (50 micrograms/kg) on plasma insulin levels were determined. In normal rats, CCK elicited a two-fold increase in plasma insulin concentration, whereas diabetic rats had a significantly lower basal insulin level which was not affected by CCK treatment. We conclude that hypnogenic and food-intake-reducing effects of exogenously administered CCK are closely associated; however, pancreatic insulin does not play a significant role in either of these effects.