The effects, of etomidate and of its fluoro analogue, R 8110, on adrenal, testicular and ovarian steroid biosynthesis were compared using cultures of guinea-pig adrenal, rat adrenal capsular, rat testicular and rat ovarian granulosa cells. At a concentration of 100 nM, etomidate inhibited the adrenal 11-hydroxylation of glucocorticoid and mineralocorticoid biosyntheses, producing a decrease in cortisol and corticosterone and an accumulation of 11-deoxycortisol and 11-deoxycorticosterone in guinea-pig adrenal and rat capsular adrenal cell suspensions, respectively. At higher concentrations (greater than 10(-6) M), etomidate also inhibited ovarian oestradiol production, testicular androgen formation and ovarian progesterone synthesis. The latter action suggests an effect on ovarian aromatase, on testicular 17 alpha/17,20-lyase activities and finally on cholesterol side-chain cleavage. The fluoro analogue of etomidate, R 8110, was ten times less potent as an inhibitor of 11-hydroxylation and affected progesterone formation only slightly in adrenal cell suspensions. Testosterone production was less affected by R 8110 than by etomidate. The increase of progestins suggests that the 17 alpha/17,20-lyase activities are the most sensitive testicular enzymatic reactions to R 8110. For inhibition of ovarian oestradiol production, R 8110 was twenty times more potent than etomidate.