Serum glucose, insulin and glucagon concentrations were measured in three patients with insulin-producing tumours of the pancreas while performing an exercise test. In contrast to the normal adrenergic inhibition of insulin release in response to exercise, plasma insulin concentration remained at a constant and high level during exercise in patients with insulinomas. Their plasma glucose concentrations fell during exercise and in the post-exercise period. No significant changes occurred in plasma glucagon concentration. An exercise test may be a useful new diagnostic tool in organic hyperinsulinism.