Clofibrate inhibits the synthesis of adrenal steroids when administered in vitro. In the present study the effect in vivo of clofibrate on adrenal steroid secretion has been investigated. Basal levels of plasma progesterone, corticosterone, aldosterone, 17-hydroxyprogesterone, 11-deoxycortisol, cortisol, dehydroepiandrosterone sulphate and dehydroepiandrosterone, and their response to ACTH 1 mg im, were not reduced by chronic administration of clofibrate 2 g per day p.o. for 8 to 34 days to 6 hyperlipidaemic men.