The effect of the pretreatment with testosterone propionate, estradiol benzoate, or a combination of both steroids, on the FSH and LH release induced by the injection of a synthetic LH-RH preparation in intact adult male rats was investigated. The pretreatment with testosterone propionate and the combination of testosterone propionate plus estradiol benzoate significantly suppressed the LH and FSH release induced by LH-RH. Estradiol benzoate alone caused a slight, not significant suppression of the response to LH-RH. In the light of the previous findings from several laboratories, it seems probable that the action of sex steroids is at least partly exerted on the pituitary gland. Thus, the sex steroids in addition to the negative feedback effect on the hypothalamus, appear to exert a modulating activity directly on the pituitary gland.
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