Rhesus monkeys with IV catheters were allowed to self-administer cocaine for 1 h/day. When responding was stable, saline or the D1 dopamine agonist SKF 81297 (SKF; 0.001-0.3 mg/kg/inj) was substituted for cocaine. At least two doses of SKF maintained responding above saline levels in all monkeys. The D1 antagonist SCH 39166 (0.001-0.03 mg/kg, IM) was then administered 30 min before sessions of self-administration of the lowest dose of SKF that maintained behavior (0.01 mg/kg/inj). SKF-maintained responding decreased in a dose-related manner, suggesting antagonism of the reinforcing effect. These results suggest that stimulation of D1 receptors can initiate a reinforcing effect and further implicate D1 receptors in the reinforcing effects of drugs that increase dopamine neurotransmission.