Double labeled in vitro incubation studies using 14C-androstenedione and 3H-testosterone have demonstrated aromatizing activity in the rhesus monkey hypothalamus and limbic system, predominantly in the former tissue. Under these conditions, no interconversion was shown between androstenedione and testosterone or estrone and estradiol. Androstenedione conversion to estrone (1.0–10.0 × 10–3%) was greater than that of testosterone to estradiol (0.2–3.6 × 10–3%), especially in the limbic system. No aromatization was demonstrated in pituitaries or cerebral cortex from these animals.