The effect of the anti-psychotic drug, haloperidol (50 mg/kg), on bar pressing in the rat was investigated in a light-reinforcement situation. Drugged animals responded significantly more for response contingent flickering light (RCF) and less for steady light (RCL) than saline injected animals. The finding is consistent with the possible arousal decreasing action of haloperidol and with the notion of an optimum level of arolsal.