The effect of acute cold exposure for 6 hours on nocturnal type II thyroxine 5'-deiodinase (5'-D) activity was studied in brown adipose tissue (BAT), Harderian gland, cerebral cortex, and pineal gland of the rat. Moreover, the effect of iopanoic acid (IOP), a potent inhibitor of 5'-D activity, on both pineal N-acetyltransferase (NAT) activity and melatonin content in rats maintained in a cold environment was also examined. Results show that acute cold exposure significantly increases 5'-D activity in BAT but not in either the pineal gland, Harderian gland, or cerebral cortex. In all tissues, the injection of IOP reduced dramatically 5'-D activity, while exposure of the animals to light at night reduced 5'-D activity in pineal gland but not in either the Harderian gland or BAT while light exposure at night increased cerebrocortical 5'-D activity. Cold exposure did not change either pineal NAT activity or the melatonin content of the gland. Finally, when pineal 5'-D activity was inhibited by IOP treatment, neither nocturnal pineal NAT activity nor melatonin content was affected.