Plasma levels of cortisol, corticosterone, deoxycortisol, deoxycorticosterone, 17OH-progesterone and progesterone were measured in a group of patients with untreated Cushing's disease and compared to levels in a similar group of patients with Cushing's disease while under treatment with the adrenalytic agent o,p'DDD. A significant increase of the deoxycortisol/cortisol and the deoxycorticosterone/corticosterone ratios was found indicating that o,p'DDD inhibits llβ-hydroxylation activity. Evidence for inhibition of adrenal 17α and 21-hydroxylation systems was not found.