The plasma concentration of C-peptide, insulin (IRI) and glucose was measured in 9 healthy subjects during insulin-induced hypoglycaemia followed by a meal. Identical observations were made in the same subjects after an equivalent period of fasting without hypolycaemia (control study). Endogenous secretion of insulin was suppressed following administration of exogenous insulin and this persisted long after the blood glucose concentration had returned to normal. After the meal the mean blood glucose rose to a peak of 8.4 +/- 0.3 mmol/l (mean +/- SEM) at 60 min and was still raised at 7.5 +/- 0.3 mmol/l at 120 min, compared with a peak value of only 5.1 +/- 0.2 mmol/l at 30 min after the meal in the control study. Following hypoglycaemia the mean plasma IRI rose from 8.3 +/- 1.3 mU/l to a delayed peak of 81.6 +/- 12.7 mU/l at 60 min and was 123.5 +/- 14 mU/l at 120 min post-prandially, compared with a peak of 72.4 +/- 0.5 mU/l at 30 min after the meal in the control study. Acute hypoglycaemia may thus induce in abnormal pattern of insulin secretion in response to a meal, with impaired carbohydrate tolerance in normal subjects.