Insulin-treated diabetic patients may show a rapid swing to hyperglycaemia after episodes of hypoglycaemia. This rebound hyperglycaemia, or Somogyi effect, is thought to be caused by the unopposed actions of hormonal antagonists to insulin secreted in response to hypoglycaemia. To test this theory a study was made of 15 patients who had 17 episodes of asymptomatic untreated hypoglycaemia (blood-glucose less than 2 mmol/l) between 11 P.M. and 3 A.M. After nocturnal hypoglycaemia, mean fasting blood-glucose concentrations at 7 A.M. ranged from 0.7-17 mmol/l and were over 7 mmol/l in 6 patients. These 6 patients with apparent rebound hyperglycaemia did not have higher levels of growth hormone, cortisol, or glucagon than those who had little or no recovery of blood-glucose. There was a close inverse correlation (r = -0.996, p < 0.001) between blood-glucose and free insulin, suggesting that hyperglycaemia, when present, was due to relative insulin deficiency in the latter part of the night. Early changes in blood-glucose after untreated hypoglycaemia seem to be primarily due to changes in free insulin rather than a response to antagonist hormones.