The present study of the circadian rhythm of locomotor activity during the summer in minks reared in a natural photoperiod showed that under laboratory conditions, this species has an essentially nocturnal behavior. But the relatively extensive activity that occurred in the light phase, as also described in the wild mink, suggests that the photoperiod is not the only factor controlling the circadian cycle. In animals exposed to constant light, the locomotor activity was concentrated in a single phase that shifted forward each day, showing that the period of endogenous rhythm is longer than 24 h under these conditions. An abrupt increase in this period of circadian rhythm was observed in one of the animals exposed to constant light.