The effect of a single oral dose of 400 mg ketoconazole, given as an 80 mg/ml suspension, on total and physiologically free (i.e. non-sex hormone-bound) testosterone and 17 beta-oestradiol has been investigated in 6 healthy male volunteers. The two steroids fell to nadir levels of 18 and 60% of their respective initial concentrations 6 hours after drug intake, and then completely recovered. Although in vitro slight displacement of testosterone from the sex-hormone binding globulin, by high doses of ketoconazole was found, the physiologically free concentration of testosterone in vivo was closely correlated with that of the total hormone, suggesting that there is no direct interference with sex-hormone binding globulin in vivo. Plasma LH and FSH were not significantly modified by treatment. The effect of ketoconazole on plasma oestradiol levels was less pronounced and was not clearly related to a block of the aromatase system, as reported in vitro.