Dopamine and norepinephrine levels were measured in individual brain nuclei in normal adult male and female rats and in adult male rats castrated on day 1 of life and in adult female rats given 1.25 mg testosterone propionate on day 4 of life. Normal adult males showed higher concentrations of dopamine than females in the caudate, tractus diagonalis and arcuate nuclei and in the median eminence. Day 1 castration of males reduced the dopamine levels of the arcuate nucleus and nucleus of the tractus diagonalis, while neonatal administration of testosterone to females elevated the dopamine concentration of the arcuate nucleus and nucleus of the tractus diagonalis. Normal adult males had higher concentrations of norepinephrine than females in the preoptic-suprachiasmatic, paraventricular, periventricular, and arcuate nuclei and in the median eminence. Neonatal endocrine manipulations (neonatal male castration and female testosterone treatment) failed to alter significantly these sex differences. The results suggest a sexual dimorphism in the catecholamine innervation of certain discrete areas of the brain and that the dopamine concentration in the arcuate and tractus diagonalis nuclei is influenced by perinatal androgen.