A B S T R A C T A variety of studies in man and animalsdemonstrate that testosterone (T) is aromatized to estradiol (E) in the hypothalamus and limbic system. These observations suggested the possibility that conversion to E is an absolute requirement for the biologic activity of T on the hypothalamic-pituitary axis. Since this hypothesis implies a common mechanism of action of these two steroids, the demonstration of divergent effects of T and E on luteinizing hormone (LH) secretion would exclude this possibility. To test this hypothesis, the actions of T and E on three separate aspects of LH release (mean LH, pulsatile LH secretion, and responsiveness to LHreleasing hormone [LH-RH]) were contrasted. T and E, infused at two times their respective production rates into normal men, reduced mean LH levels similarly during 6 h of steroid infusion and for 6 h thereafter. However, these steroids exerted different effects on pulsatile secretion. E reduced the amplitude of spontaneous LH pulses from pre-and postinfusion control levels of 75+14 and 68±5.6% (SEM) to 39±5.7%. In contrast, T increased pulse amplitude to 96+14% and decreased pulse frequency from basal levels of 3.4+0.31 to 1.8± 0.31 pulses/6 h.The site of suppressive action was determined by administering 25 lAg of LH-RH to the same men during T and E infusions and during three additional control periods without steroid administration. LH-RH produced similar 170-190% increments in serum LH during the three control periods and during T infusion. In contrast, E markedly blunted (76±31%, P <0.005) the LH response to LH-RH. Under the conditions of acute steroid infusion at doses (utilized in these experiments) pro- ducing similar inhibition of mean LH, E but not T acted directly on the pituitary to diminish LH-RH responsiveness.As further support that androgens can act without conversion to estrogens, the effects of a nonaromatizable androgen, dihydrotestosterone (DHT), on mean LH levels were studied. DHT, infused at the same rate as T, suppressed mean LH to a similar but somewhat greater extent than T. Since T and E produced divergent effects on LH secretion and a nonaromatizable androgen, DHT, suppressed mean LH, aromatization is not a necessary prerequisite for the action of androgens on the hypothalamic-pituitary axis.