Squirrel monkeys responded under a multiple fixed-interval (FI) fixed-ratio (FR) schedule of stimulus-shock termination. Benztropine mesylate (0.03-1.7 mg/kg), bupropion HCl (0.3-5.6 mg/kg), mazindol (0.01-0.3 mg/kg), and nomifensine maleate (0.1-1.0 mg/kg) markedly increased responding under the FI schedule, but not under the FR schedule. Mazindol was about three-times more potent than nomifensine and ten-times more potent than bupropion. Benztropine and mazindol were about equal in potency. The order and relative magnitude of potency differences for mazindol, nomifensine, and bupropion are similar to those reported by others for in vitro inhibition of dopamine uptake in rat striatum, but the relative potency of benztropine was greater in these behavioral experiments than expected from its potency in inhibiting dopamine uptake.