Abstract. The aim of the present experiments was to analyze whether the inhibitory effect exerted by testosterone on FSH release might be mediated by the intracerebral transformation of the hormone into oestrogenic metabolites. Advantage has been taken of the availability of the potent antioestrogen tamoxifen. Two series of experiments have been performed. In the first one, adult male rats have been castrated and submitted, beginning immediately after surgery, to a 6-day treatment with testosterone propionate (2 mg/rat/day), tamoxifen (50 or 200 μg/rat/day) or testosterone propionate (2 mg/rat/day) plus tamoxifen (either 50 or 200 μg/rat/day). In the second experiment, adult male rats have been castrated and submitted to the same 6-day treatments, beginning 4 weeks following orchidectomy. In both experiments, the animals were killed 24 h after the last injection, and serum levels of FSH and LH have been measured by radioimmunoassays. The results have clearly shown that, in both experiments, the administration of testosterone results in a significant decrease of serum FSH and in a total suppression of LH release. The administration of tamoxifen, in either dose, does not modify the elevated serum FSH and LH levels present in the orchidetomized animals, and does not antagonize the inhibitory effect on FSH and LH secretion exerted by the concomitant treatment with testosterone propionate. It is concluded that testosterone inhibits FSH secretion in orchidectomized rats acting as such, and not following aromatization to oestrogens.