To test the hypothesis that time of day influences the locomotor effects of cocaine in rats, we measured short- and long-term sensitization to cocaine at five different Zeitgeber times (ZT). Short-term sensitization was expressed equally at all ZTs tested. Long-term sensitization was expressed only at dark onset, suggesting a possible influence of melatonin. Exogenous melatonin given immediately before cocaine challenge slightly enhanced long-term sensitization. Taken together, the present results support a time-of-day influence on locomotor sensitization to cocaine.