A new antagonist of the peripheral cholecystokinin receptor, L-364,718, was found to block the reductions in food intake and exploratory activity induced by intraperitoneal administration of cholecystokinin octapeptide sulfate. L-364,718 significantly reversed the cholecystokinin-induced reduction in feeding at doses of 10 micrograms/kg - 10 mg/kg i.p. L-364,718 significantly reversed the cholecystokinin-induced reduction in exploratory activity at doses of 500 ng/kg - 10 mg/kg i.p. The time course of antagonist activity of L-364,718 was immediate to 90 minutes after intraperitoneal administration. L-364,718 had no significant effect on food intake or exploratory activity when administered alone, over the dose range of 100 ng/kg-10 mg/kg i.p. This compound appears to be at least one hundred times more potent than proglumide or benzotript as an antagonist of the behavioral effects of peripherally administered cholecystokinin.