The role of serotonin in the prolactin, growth hormone, ACTH and cortisol responses to hypoglycaemia has been investigated in normal subjects using a selective serotonin (5HT2) receptor antagonist, ketanserin. Circulating concentrations of these hormones were measured after administration of insulin (0.1 units/kg body weight iv) to eight normal male subjects with and without simultaneous iv ketanserin (10 mg). Plasma glucose fell to less than 2.0 mmol/1 in all subjects and was unaffected by ketanserin. Ketanserin induced a 50% decrease in the serum prolactin response to hypoglycaemia, 45 and 60 min after administration of insulin (increase in serum prolactin at 60 min: 1145 +/- 295 mU/l without ketanserin; 558 +/- 176 mU/l with ketanserin, P less than 0.05). The peak ACTH response was reduced by 30% (95.3 +/- 33.6 ng/l without ketanserin; 60.0 +/- 22.9 ng/l with ketanserin, P less than 0.05) but the plasma cortisol response was not significantly altered. The serum growth hormone response was unaffected by serotonin blockade. These findings suggest that serotonin, probably acting through 5HT2 receptors, is involved in the stimulation of prolactin and ACTH release but not in the release of growth hormone, during insulin induced hypoglycaemia.