Herrnstein's matching equation was used to analyze drug effects on performance in random interval reinforcement schedules. Pimozide caused effects compatible with both motor and motivational impairments, in a 5-component multiple schedule, a 3-schedule 3-day cycle (ALT-3), and a 2-schedule 2-day cycle (ALT-2). However, at low doses, both sulpiride and SCH-23390, tested in the ALT-3 and ALT-2 procedures, caused effects compatible with selective motivational impairments. In experiments using the non-multiple schedules, motivational effects increased during the course of the experimental session, under all three drugs. The interpretation of "motor" and "motivational" deficits in the ALT-2 procedure was validated by experiments in which the response-force and deprivation level were systematically varied. The results support the view that dopamine may be involved in the maintenance of rewarded behaviour, but not differentially implicate the D1 or the D2 receptor subtype.