SUMMARY The role of the thyroid in cold-induced atrophy of the genital accessory organs and the protection by treatment with testosterone against such atrophy have been evaluated in intact and castrated male rats. Treatment with thyroxine or exposure to cold reduced the weight and some of the activities of the genital accessory organs in intact but not in castrated rats. Testosterone propionate implants not only protected these organs from the cold-induced atrophy but also stimulated them beyond the level of intact controls kept at room temperature. The weight and some functions of the genital accessory organs of testosterone-treated castrated rats were decreased after cold exposure. These changes were more pronounced when thyroxine was administered simultaneously. The results suggest that the cold-induced atrophy of the accessory organs is due to the release of excess thyroid hormone and that implants of testosterone propionate can prevent this atrophy.