The effects of amphetamine and pimozide were studied in rats performing on variable-interval (VI) schedules of reinforcement. In experiment 1, a five-component multiple VI schedule was used; in experiments 2 and 3, two VI schedules were presented on alternate days; a fourth experiment was carried out to validate the two VI methods. The Herrnstein matching law was used to distinguish between changes in reinforcer efficacy and changes in motor capacity. In both procedures, amphetamine, at 0.5 mg/kg, caused changes compatible with an increase in reinforcer efficacy with no change in motor capacity; higher doses (only tested in the multiple schedule) appeared further to increase reinforcer efficacy and also to decrease motor capacity. In the multiple schedule, pimozide caused changes compatible with either a decrease in reinforcer efficacy or a decrease in motor capacity, depending on the order of presentation of the schedule components; in the alternating schedule, pimozide had both effects. These discrepancies appeared to be due to changes in the effect of pimozide over time, which could not be explained either by satiation or by a gradual onset of drug action.