Rats received an i.p. injection of cholecystokinin-octapeptide sulfate ester (CCK; 4, 10 or 50 micrograms/kg) or physiological saline at dark onset, and the 24-h sleep-wake cycle (12-h-dark and 12-h-light phases), spontaneous motor activity and brain temperature (Tbr) were recorded. EEG activity was studied through spectral analysis for 2.5 h, and food intake was measured at the end of postinjection hour 1. In response to CCK, non-REM sleep increased at the expense of wakefulness, and the sleep-promoting effect was substantiated by an increase in EEG slow-wave activity. Motor activity, Tbr and food intake decreased. The effects vanished in postinjection hour 2; the diurnal rhythms were not modified. The changes varied as a function of the dose: the effects were significant following 10 micrograms/kg, and even higher in response to 50 micrograms/kg CCK. The results indicate that i.p. CCK definitely promotes non-REM sleep. This effect may belong to the behavioral sequence elicited by the peptide, which is often attributed to satiety. As evidenced by the reduction of Tbr, CCK also exerts strong autonomic actions, which might interfere with the behavioral responses.