Three squirrel monkeys performed under a 5-min fixed-interval schedule of food presentation with licking a water-filled tube as the required operant response. All subjects developed the characteristic fixed-interval pattern of responding: an initial period of little or no responding followed by acceleration of responding to a final rate that is sustained until reinforcement. Over a range of doses, d-amphetamine had a dose-related enhancing effect on the overall rate of licking; higher doses decreased the overall rate. Analysis of the effects of d-amphetamine on the different rates within successive 30-see periods of the interval showed that all doses in one subject and the higher doses in the two others changed the output of the licking behavior in a rate dependent way. Over a range of doses d-amphetamine not only increased the rate of licking but also the intake of water directly proportional to the number of responses. After high doses, which still increased the rate of responding, the water intake per lick decreased. The effects of d-amphetamine on licking behavior maintained under a fixed-interval schedule of food presentation were similar to the effects of amphetamine on other operant responses, such as pressing a lever or pecking a key. Schedule-controlled licking is influenced by d-amphetamine differently from drinking which is not under the control of explicit experimental consequences.