To assess whether exercise-induced hypoglycaemia could be prevented by interruption of insulin infusion (3 h) we studied diabetic patients treated with continuous subcutaneous insulin infusion (CSII). The studies were performed in 7 insulin-dependent diabetics (aged 31.4 +/- 4.8 (mean +/- SD) years, duration of diabetes 16.9 +/- 5.4 years), after an overnight fast and in the afternoon, 4 h after the last pre-meal bolus injection (exercise and control period). Bicycle exercise (45 min at 60% of maximum oxygen consumption) was started 30 min after the insulin infusion was stopped. During exercise there was a more pronounced decline in blood glucose in the afternoon (2.2 +/- 0.3 mmol/l, mean +/- SEM) than in the morning (1.4 +/- 0.4 mmol/l) (p less than 0.01). This corresponded to higher mean levels of free insulin during exercise in the afternoon (20 +/- 4.5 mU/I vs 12.0 +/- 1.0 mU/l, in the morning). Interruption of insulin delivery for 3 h resulted in a moderate increase of blood glucose, a gradual decrease of free insulin, and a moderate increase in free fatty acids and beta-hydroxybutyrate. During exercise in the afternoon 3 diabetics suffered from symptomatic hypoglycaemia (BG less than 2.8 mmol/l). In contrast with most of the other patients they showed no decline of free insulin during exercise. Thus even after interruption of basal rate insulin infusion moderate postprandial exercise may lead to hypoglycaemia if there is relative hyperinsulinism.