We have observed previously that systemic treatments or local implants of testosterone (T) suppress dopamine (DA) turnover in the preoptic area-anterior hypothalamus of male rats. In the present study, we sought to identify discrete regions innervated by the incertohypothalamic DA system and the tuberoinfundibular DA system which respond to T replacement. Adult male rats were orchidectomized and immediately treated with either empty (controls) or T-containing Silastic implants. After 14 days, animals from each group were treated with alpha-methylparatyrosine and killed 0, 45, and 90 min later for analysis of DA turnover in eight microdissected brain regions. The T implants produced an increase in serum T and 5 alpha-dihydrotestosterone and reduced serum LH to concentrations observed in intact male rats without affecting serum PRL levels. Serum levels of T within the physiological range caused a profound decrease in the DA turnover rate in the medical preoptic nuclei and anterior hypothalamic nuclei without influencing DA activity in the periventricular nuclei, the site of perikarya for these DA projections. In contrast, DA turnover in the median eminence was increased 3-fold by T treatment, while DA activity in the arcuate nuclei, the locus of cell bodies of the tuberoinfundibular DA system, was not affected. These studies reveal disparate effects of T on the terminal fields of the two DA systems. While augmentation in the median eminence DA activity may participate in the negative feedback effects of T on gonadotropin secretion, the T-induced suppression of DA turnover in the medial preoptic nuclei and anterior hypothalamic nuclei may well be involved in androgen-dependent aggression and copulatory behavior.