To understand the role of central aromatization in feedback regulation of LH in nonhuman primates, we treated adult male cynomolgus monkeys with the aromatase inhibitor, 1,4,6-androstatriene-3,17-dione (ATD). We measured LH, testosterone (T), and ATD in systemic sera of blood samples drawn on a diurnal schedule (0900 and 2100 h). Each animal was bled for 4 pretreatment days from a femoral catheter after which they were divided into the following treatment groups: castrated (Cx), n = 2; Cx + T, n = 6; Cx + T + ATD, n = 6; Cx + ATD, n = 3; and sham operated + ATD, n = 3. Silastic capsules or packets containing T or ATD, respectively, were placed sc between the scapulae at the time of Cx or sham treatment. In T-treated animals, T (20 micrograms/kg body weight) dissolved in propylene glycol was injected im at 2100 h to mimic the diurnal rise of T observed in nonhuman primates. Animals were bled for 2 weeks after which they were killed, and selected brain areas were analyzed for aromatase activity and cytosolic and nuclear androgen receptors. Animals treated with ATD had significantly reduced levels of aromatase activity in selected regions of the hypothalamus, preoptic area, and the amygdala (P < 0.05). Even though ATD inhibited brain aromatase activity, it did not prevent the negative feedback actions of T on LH secretion after Cx. In addition, ATD by itself inhibited LH secretion after Cx and activated brain androgen receptors. These latter effects of ATD seemed to have been mediated through a metabolite. In sham-operated intact males, ATD produced variable surges of LH that were accompanied by elevations of T in the systemic circulation. These differential effects of ATD in intact vs. castrated animals demonstrate the importance of selecting the proper model system to study LH control mechanisms. In the intact animal, aromatization seems to play a role in regulating LH secretion, but the postcastration rise of LH seems to be regulated differently.