The effect of the cyclo-oxygenase inhibitor, indomethacin, on hypothalamic-pituitary-adrenal function was investigated in five normal males. Baseline 0800 and 1600 h plasma ACTH and cortisol, plasma ACTH and 11-deoxycortisol responses to metyrapone, and plasma cortisol responses to dexamethasone and exogenous ACTH were not affected by indomethacin. However, the area under the curve for plasma ACTH after iv injection of regular insulin (0.1 U/kg) was significantly decreased during indomethacin administration (control, 88.0 +/- 32.6 cm2, mean +/- sd; indomethacin, 47.6 +/- 23.1, P less than 0.01). Plasma cortisol levels were decreased only at 30 min. Our results support the hypothesis that prostaglandins or any of their precursors play a role in the hypothalamic control of ACTH secretion and indicate that evaluation of hypophyseal ACTH secretory capacity by means of insulin-induced hypoglycemia may yield abnormal results in patients receiving indomethacin.