Steroid regulation of pituitary LH and FSH secretion in the male may be mediated via the direct action of testosterone, or via the conversion of testosterone to either dihydrotestosterone (DHT) or estrogenic steroids. The site of testosterone conversion to these metabolites may be peripherally and/or centrally located. The purpose of this study was to investigate the importance of the conversion of testosterone to estradiol in the control of gonadotropin secretion using the aromatase inhibitor aminoglutethimide (AG). In testosterone-implanted orchidectomized dogs, AG prevented the ability of testosterone to inhibit the post-orchidectomy rise of LH, despite the presence of normal plasma concentrations of both testosterone and estradiol. These data suggest that aromatization of testosterone to estradiol is a necessary step in the regulation of LH secretion. Furthermore, a lack of change in the peripheral concentrations of estradiol in the presence of AG led us indirectly to conclude that inhibition of aromatization occurred centrally rather than peripherally. In the absence of AG, testosterone was unable to maintain intact levels of FSH in the orchidectomized dog for more than six days. Thus, it still remains to be demonstrated whether steroids are of major importance in the regulation of FSH secretion.