SUMMARY The in-vitro conversion of testosterone to 17β-hydroxy-5α-androstan-3-one (5α-dihydrotestosterone, DHT) and 3α, 17β-dihydroxy-5α-androstane (3α-androstanediol, DIOL) in pituitary and slices of brain regions was compared between male and female rats. Intact pituitaries from male rats formed 2·5 times more DHT and 1·5 times more DIOL than those of females. A small sex difference was also detected in the hypothalamus, males again being higher than females. No sex differences could be detected in other brain regions. However, DHT formation in the brain was regionally differentiated with higher conversion rates in hypothalamus than in cortex, hippocampus, amygdala, pineal gland or cerebellum. The highest transformation, however, was found in the mid-brain. Metabolism in the pre-optic area was as low as that in the cortex. 5α-Dihydrotestosterone and DIOL formation in the pituitary increased several-fold after gonadectomy in both sexes and the sex difference disappeared. Little or no increase occurred after thyroidectomy or adrenalectomy. The increase in pituitary DHT formation after gonadectomy could be attenuated or prevented both by treatment with testosterone propionate and with oestradiol benzoate. Replacement therapy, particularly with oestradiol benzoate, gave rise to a sex difference reminiscent of that of normal animals. No significant change in pituitary DHT formation occurred in adult females which had been treated with testosterone propionate on the 4th day of life. The results suggested a close relationship between DHT formation and activity of gonadotrophin secretion, particularly at the level of the pituitary.