Plasma testosterone, FSH, and LH levels were measured by RIA in sham-castrate control rats and in castrate male rats with testosterone-containing Silastic capsule implants to evaluate the relative contribution of testosterone and inhibin in maintaining normal FSH secretion in vivo. Silastic capsules maintaining normal testosterone levels maintained normal levels of both FSH and LH over the 5-day course of these studies. The data suggest that testosterone or its metabolites can account for at least 78%, and possibly all, of the FSH-suppressing activity of the testis. The data do not support an obligatory role for other testicular factors, such as inhibin, in the regulation of plasma FSH when normal testosterone levels are maintained.