The influence of photoperiod on resting oxygen consumption and nonshivering thermogenesis of winter-acclimatized kangaroo rats was determined by exposing freshly captured animals to 10-week controlled photoperiod treatments. Three treatments were employed: 1) long days (LD) = 16L:8D, 2) short days (SD) = 8L:16D, and 3) short days interrupted with 15 min light at midnight (SDL). Thermoneutral oxygen consumption was uninfluenced by treatment, and posttreatment values were not different from pretreatment values. Below thermoneutrality, mean values for oxygen consumption and norepinephrine(NE)-induced thermogenesis each were significantly greater in the SD group than they were in either the LD or SDL groups. Furthermore, oxygen consumption of LD and SDL groups, and NE-induced thermogenesis of LD and SDL groups were indistinguishable. Posttreatment oxygen consumption and NE-induced thermogenesis of the SD group were not different from pretreatment values. These results indicate that thermogenic capacities of kangaroo rats are strongly influenced by photoperiod. The similarities of responses by the LD and SDL groups indicate that the length of the dark period is of importance, and indirectly support suggestions that the pineal gland, through its nocturnal secretion of melatonin, may be involved in the transduction of photic information.