The influence of acute and chronic light/dark cycle alteration on morphine-induced conditioned place preference was evaluated in mice. In chronic experiments, morphine (1-10 and 2.5-10 mg/kg) induced conditioned place preference in animals maintained on 12/12 and 6/18 light/dark cycles, respectively. In mice maintained on 18/6 light/dark cycle, however, morphine produced conditioned place preference only by the dose of 10 mg/kg. This latter effect was abolished by chronic daily administration of melatonin (10, 20 mg/kg). Moreover, our data showed that both abrupt increase and decrease in photoperiod on the day before testing reduced the ability of morphine to produce place conditioning. A single melatonin (10 mg/kg) injection reversed the attenuating effect of a 6-h delay in the dark cycle on morphine conditioned place preference.