We investigated basal and ACTH stimulated levels of cortisol, corticosterone, 17 alpha-hydroxyprogesterone, 11-deoxycortisol and 11-deoxycorticosterone as well as plasma levels of ACTH before and during the oral administration of ketoconazole in five patients with Cushing's syndrome (3 with bilateral adrenal hyperplasia, 1 with adrenal adenoma and 1 with adrenal carcinoma) and in three controls. The influence of ketoconazole on the transformation of 3H-17 alpha-hydroxyprogesterone to 3H-11-deoxycortisol and 3H-cortisol and of 3H-11-deoxycortisol to 3H-cortisol as well as of 3H-11-deoxycorticosterone to 3H-corticosterone was also examined in slices or homogenates of normal and hyperplastic adrenal tissue from four patients. Ketoconazole induced a rise of 11-deoxycortisol and 11-deoxycorticosterone, but not of cortisol and inconsistently of corticosterone which were increased by ACTH. Thus the ratio 11-deoxycortisol/cortisol rose more after ketoconazole than after ACTH and the ratio 11-deoxycorticosterone/corticosterone rose after ketoconazole but fell after ACTH. Plasma ACTH levels were stimulated 2-50 fold by ketoconazole. Incubation studies of adrenal tissue slices with 3H-17 alpha-hydroxyprogesterone showed that ketoconazole inhibited the transformation of 3H-17 alpha-hydroxyprogesterone to 3H-cortisol but not to 3H-11-deoxycortisol so that the ratio 3H-11-deoxycortisol/3H-cortisol increased 15-80 fold. After incubation of adrenal slices with 3H-11-deoxycortisol or 3H-11-deoxycorticosterone and ketoconazole, a 2-260 fold increase of the ratios 3H-11-deoxycortisol/3H-cortisol and 3H-11-deoxycorticosterone/3H-corticosterone were also found.