For a single-dose metyrapone test, from 2 to 3 gm of the drug were given at midnight. Blood for plasma deoxycortisol and adrenocorticotropic hormone determination was obtained at 8 AM the next morning. Urine for assay of 11-deoxycorticosteroids was also collected from midnight to midnight before and after metyrapone administration. Using any of the three determinations, we found no overlap between 30 normal subjects and 11 patients with pituitary insufficiency. The results are comparable to those obtained with the standard metyrapone test (750 mg every four hours, six times). We conclude that the single-dose metyrapone test is a simple and reliable test of pituitary function.