Daily torpor in P. leucopus is an endogenous circadian rhythm which employs photoperiod as a zeitgeber. Mice maintained under constant darkness exhibited a free-running rhythm with a period of 23.52 h. Entrainment of this free-running rhythm occurred when a LD = 9:15 photoperiod was provided.Constant light seriously disrupts this rhythm in that torpor ceased in 50% of the mice examined, and the mice which continued to become torpid exhibited aberrations in the rhythm. Under constant light, the period of the free-running rhythm was 25.00 h.There is a substantial association between the depth and duration of torpor and advance of entry in the free-running rhythm under constant dark (r =. 7; d.f. = 8).