The adrenal gland of castrated adult male rats metabolized [3H]dehydroepiandrosterone in vitro to delta 4-androsten-3,17-dione (4AD), testosterone, dihydrotestosterone (DHT) and 5 alpha-androstane-3,17-dione (5 alpha AD). Despite the low testosterone values, DHT and 5 alpha AD were higher 30 and especially 60 days after castration, with raised 4AD:testosterone and decreased testosterone:DHT ratios. The 5 alpha-reductase activity thus appears to increase with time after castration. Fourteen days after castration, 4AD was the only metabolite that was raised compared with intact animals, and testosterone was comparable in sham-operated and castrated rats. The administration of testosterone propionate to castrated rats restored testosterone values to those of intact rat adrenals, whereas 4AD values were greater. The administration of dihydrotestosterone propionate also yielded higher levels of 4AD, in the presence of a lower testosterone value. After administration of oestradiol benzoate, 4AD values were lower especially compared with the other hormone-treated groups, and there was an unexpectedly high testosterone value. These data indicate that the adrenal gland contributes to the production of androgens, as previously noted by Andò, Canonaco, Beraldi et al. (1988) who showed increased plasma 4AD and testosterone levels in adult male rats 30 days after castration. Furthermore, adrenal androgen production in castrated animals is differentially regulated by sex steroids.