Eighty-six patients received standardized intravenous methopyrapone (SU-4885) in tests designed to evaluate ACTH reserve. SU-4885 ditartrate, 30 mg/kg in 1000 ml normal saline, was infused during a 4-hour period between 8 and 12 A.M. Twenty-four-hour urine samples collected from the beginning of the infusion were then analyzed for changes in 17-ketogenic steroids. Compared to a normal rise in 24-hour urinary 17-ketogenic steroids of 12 to 15 mg, patients with hypo-thalamic lesions showed a markedly reduced response, as did those with nonfunctioning pituitary tumors generally. However, when associated with acromegaly, the response was usually normal. In the presence of Cushing''s syndrome, a normal or greater than normal response was noted in patients with overactivity of the adrenal cortices in the absence of demonstrable tumors. In contrast, 2 cases of carcinoma and 2 cases of adenoma failed to show a significant rise in 17-ketogenic steroids, making this a useful differential point. Adrenal insufficiency and hypothyroidism, as well as some cases of uncontrolled hyper-thyroidism, seemed to decrease the response to the intravenous SU-4885 test markedly. Four cases with a poor preoperative SU-4885 response showed a significant rise in urinary 17-ketogenic steroids in surgery, demonstrating that a negative response to SU-4885 does not necessarily call for corticoid coverage during operation.