The aim of this study was to investigate the influence of short-term submaximal postprandial exercise on plasma glucose concentrations in tightly controlled insulin-dependent diabetic patients treated by means of continuous subcutaneous insulin infusion (CSII). Two hours after breakfast, five diabetic patients and five healthy control subjects followed this protocol: 30 min of mild exercise, 30 min rest, 30 min of moderate exercise, 150 min rest. Serial determinations of plasma glucose, free insulin, and growth hormone (GH) were made. Similar control studies without exercise were also performed. In the diabetic patients, analysis of variance and covariance did not reveal any significant difference between the 2-h postbreakfast concentrations of plasma glucose and free insulin and postexercise values. A significant GH increase was observed after the exercise periods. Plasma glucose and insulin concentrations throughout the exercise study were not significantly different from the control study concentrations. Plasma free insulin concentrations of the diabetic patients were higher than the concentrations of healthy subjects. We conclude that CSII-treated, tightly controlled, insulin-dependent diabetic patients performing short-term mild and moderate exercises 2 and 3 h after breakfast do not have a high risk of hypoglycemia in spite of mild hyperinsulinemia.