The spontaneous motor activity of rats was measured following administration of cocaine alone and in combination with the centrally acting alpha 1-antagonist prazosin. Cocaine alone (18-42 mg/kg) increased motor activity in a dose-related manner. At doses of 1 and 1.8 mg/kg, prazosin attenuated the increases in motor activity produced by cocaine. In rats responding under a fixed-ratio discrimination procedure, cocaine (10-32 mg/kg) produced dose-dependent increases in percent errors and decreases in overall response rate. Across a range of doses (0.32-3.2 mg/kg), prazosin failed to antagonize the effects of cocaine on responding under the discrimination procedure. Rather, the combined effects were frequently greater than those obtained with cocaine alone. The data suggest that in rats activation of alpha 1-adrenergic systems may mediate the effects of cocaine on motor activity but not on schedule-controlled behavior.