The effect of cycloheximide, an inhibitor of protein synthesis, on the response of the rat adrenal to ACTH was studied. Cycloheximide blocks corticosteroidogenesis in vivo and in vitro, but does not affect the increase in adrenal blood flow in vivo. When the corticosterone production of adrenal slices was studied after ACTH stimulation in vivo, it was found that adrenal slices from rats pre-treated with cycloheximide, secreted corticosterone just as efficiently as adrenal slices from control animals. It is concluded that cycloheximide does not block the primary action of ACTH but that it inhibits subsequent enzymatic processes taking place in the mitochondria. The hypothesis is put forward that the increase in adrenal blood flow induced by ACTH might be due to prostaglandins which could be formed from unsaturated fatty acids released by the cleavage of cholesterol esters.