Experiments with Sprague-Dawley rats showed that after injection of estradiol benzoate, testosterone propionate, and progesterone the weight of the thyroid gland and the thyroxinebinding capacity (TBC) of the blood plasma proteins were higher in females than in males. The level of total thyroxine (TT4) and the free thyroxine (FT4) index were lower in females than in males after injection of testosterone propionate and progesterone. In males estradiol benzoate raised the TT4 and FT4 levels and in females it led to an increase in the TT4 and TBC content. Testosterone propionate lowered the sensitivity of the thyroid gland to thyrotropic hormone. Progesterone had a stimulating action on the thyroid gland and caused a greater increase in the intensity of its function and its more rapid exhaustion in females than in males.