Seven normal male volunteers were given a 2 h intravenous infusion of insulin (2 mu kg-1 min-1). During the first hour, in which nomoglycaemia was maintained by glucose infusion, no detectable increase in growth hormone (GH) secretion occurred. When controlled symptomatic hypoglycaemia was produced during the second hour of the insulin infusion a brisk rise in plasma GH concentration was observed in all the subjects studied. Since it was possible that the falls in the concentration of plasma free fatty acids (FFA) occurring during infusion of insulin also acted as a stimulus to GH release the study was repeated in four subjects but normoglycaemia was maintained throughout the entire 2 h period of insulin infusion. In these experiments there was no rise in plasma GH concentration although a similar fall in plasma FFA was produced. These results clearly indicate that hypoglycaemia per se was the important stimulus to GH secretion and not hyperinsulinaemia or a lowering of plasma FFA. Furthermore there appeared to be a threshold hypoglycaemic stimulus to GH secretion. In no subject was a rise in plasma GH seen without a fall in plasma glucose greater than 1.4 mmol/l. Prolonged mild hypoglycaemia did not stimulate GH secretion.