Seven well-trained adolescent boys with juvenile diabetes mellitus were studied. They were non-ketotic and performed a 60 min exercise in the afternoon at an average intensity of 58% of maximal working capacity. Blood glucose, blood lactate, plasma levels of insulin, C-peptide and 3-hydroxybutyrate, oxygen uptake and respiratory quotient were determined. The perception of exertion was scored as rate of perceived exertion. Healthy athletes of the same age were controls. Blood glucose, monitored continuously, decreased successively after 10-15 min of exercise with a significant correlation to the pre-exercise glucose value. Blood glucose decreased during exercise to the same level or below that of controls. Despite hypoglycemia with values of 2-3 mmol/l, the exercise continued. Plasma insulin was unchanged. The 3-hydroxybutyrate level was significantly higher in the diabetics, increased somewhat during exercise and still more during the recovery period. The diabetic subjects had significantly higher systolic blood pressure and heart rate during exercise despite a relatively equal work intensity and lower absolute workload. The rate of perceived exertion increased normally during exercise without interference of decreasing blood glucose.