Proglumide (0.02 mg/kg), a cholecystokinin antagonist, was administered to rats either together with or without morphine (0, 5, 15, or 45 mg/kg). Whereas proglumide in the absence of morphine showed a trend towards enhanced behavioral activation, it potentiated the hypokinesia induced by morphine. These results are consistent with the hypothesis that endogenous cholecystokinin tonically antagonizes opiate modulation of motility, irrespective of whether such modulation is produced by opiates and endogenous or exogenous origin.