In normal mature male rats, the daily injection of 2 mg of testosterone propionate (TP) for 10 days prior to and during the estimation of the thyroid hormone secretion rate (TSR) resulted in a significant increase of 25%. Gonadectomy of similar rats resulted in a significant reduction of TSR of 12%. When gonadectomized rats were injected with 2 mg TP daily, the TSR was increased 21%, a significant increase in comparison with the normal TSR and a significant increase of 35% in comparison with the mean TSR after castration.