The effect of low environmental temperatures on the activity of the thyroid gland was investigated in the golden hamster. No histological hyperactivity was observed in cold-exposed animals but I 131 uptakes and conversion ratios in cold-exposed hamsters were significantly greater than control animals. Chromatograms of thyroid hydrolysates of cold-exposed and control hamsters were similar. No significant differences in the protein-bound iodine level and the red cell uptake of I 131 labeled triiodothyronine were demonstrated between these two groups. The half-life of radioactive thyroxine was significantly shorter in cold-exposed than control hamsters. It is concluded that the thyroid gland does not play an important causative role in the initiation of hibernation. In hibernating hamsters, there was an extremely low incorporation of I 131. Chromatograms of thyroid hydrolysates revealed that MIT and DIT were present while T 3 and T 4 were not detectable. No significant differences were observed in the protein-bound iodine levels of hibernating and control hamsters. The red cell uptake of hibernating hamsters was significantly lower than control hamsters. In vitro incubation of thyroid glands at 37°C revealed a significantly lower uptake of I 131 in tissue from hibernating hamsters. It is suggested that the reduced activity of the thyroid gland observed in vivo during hibernation is due to the direct effect of lowered temperature on the metabolism of the thyroid gland and the decreased secretion of thyrotropin by the pituitary gland.