The response of growth hormone, cortisol, and catecholamines to hypoglycaemia produced by a continuous intravenous infusion of insulin was investigated in 10 normal subjects and 15 patients with pituitary disease. The insulin infusion rate was started at 2 U/hour for adolescents, 4 U/hour for adults, and 6 U/hour for patients with acromegaly. If required the rate was increased during the test depending on changes in blood glucose, measured by a Reflomat with low reading glucose oxidase strips. Stopping the infusion when the blood glucose concentration had fallen to 2.0 mmol/l (36 mg/100 ml) resulted in a maximum further fall of 0.7 mmol/l (13 mg/100 ml) and a subsequent spontaneous rise in blood glucose concentration. The rise was identical in normal subjects and in patients with hypopituitarism, further evidence that pituitary hormones--in contrast to glucagon and catecholamines--are relatively unimportant in the recovery from hypoglycaemia. The only patient who required intravenous glucose to restore normoglycaemia was a patient with longstanding insulin dependent diabetes. A comparison with the conventional bolus injection test showed that continuous intravenous insulin infusion was more reliable in producing adequate but not excessive hypoglycaemia and the hormone responses were equivalent. The continuous intravenous insulin infusion may offer particular advantages in the investigation of growth hormone deficiency.