We demonstrated previously that testosterone regulates aromatase activity in the anterior/dorsolateral hypothalamus of male rhesus macaques. To determine the level of the androgen effect, we developed a ribonuclease protection assay to study the effects of testosterone or dihydrotestosterone (DHT) on aromatase (P450_AROM) mRNA in selected brain areas. Adult male rhesus monkeys were treated with testosterone or DHT. Steroids in serum were quantified by RIA. Fourteen brain regions were analyzed for P450_AROM mRNA. Significant elevations of its message over controls (P < 0.05) were found in the medial preoptic area/anterior hypothalamus of both androgen treatment groups and the medial basal hypothalamus of the testosterone-treated males. Other brain areas were not affected by androgen treatment. We conclude that testosterone and DHT regulate P450_AROM mRNA in brain regions that mediate reproductive behaviors and gonadotropin release. The P450_AROM mRNA of other brain areas is not androgen dependent. Brain-derived estrogens may also be important for maintaining neural circuitry in brain areas not related to reproduction. The control of P450_AROM mRNA in these areas may differ from what we report here, but it is equally important to understand the function of in situ estrogen formation in these areas.