In seven normal volunteers, the basal and arginine-stimulated levels of plasma growth hormone, glucose, glucagon, and free fatty acids and serum insulin was determined during placebo and during indomethacin 25 mg four times daily. Basal plasma growth hormone was higher during indomethacin (5.4 +/- 1 vs 1.7 +/- 2 ng/ml, p less than 0.005), as was the mean integrated response to intravenous arginine (40 +/- 14 vs 6 +/- 4 ng.min/ml, p less than 0.05). Despite lack of effect on plasma glucagon, indomethacin decreased the insulin response to arginine. The fall in free fatty acids during arginine infusion was decreased by indomethacin, which may in part be due to increased plasma growth hormone levels. In view of the many patients treated with indomethacin, the effect on growth hormone may be of clinical importance.