The effects of pharmacological manipulations on responding under extinction conditions were determined in baboons using a schedule of reinforcement that modeled food acquisition and food consumption. Responding during the initial acquisition component was reinforced by stimuli paired with food, while responding during the latter consumption component was reinforced with food. Certain sessions began with a 7-h extinction phase, where responding in both components produced only the paired stimuli. Dexfenfluramine (DFEN) decreased responding during extinction. Diazepam (DZP) increased responding during extinction. Low doses of amphetamine (AMPH) increased responding during extinction. Thus, DZP and AMPH increased and DFEN decreased the conditioned reinforcing effects of stimuli paired with food.