The effects of 4 months' physical exercise on the metabolic control in non-insulin-dependent diabetes were studied in 25 patients divided randomly into exercise (n = 13) and control (n = 12) groups. In the exercise group glycosylated hemoglobin A1 fell from 9.6-8.6% (p less than 0.01) and the 2 hour plasma glucose in oral glucose tolerance test decreased from 19.7-16.5 mmol/l (p less than 0.01). The mean fall in fasting plasma glucose from 11.8-10.5 mmol/l was not statistically significant. The responses of plasma insulin and C-peptide to oral glucose increased significantly during the exercise period. However, patients with initially poorest metabolic control were not able to improve their physical fitness and did not show significant improvement in metabolic control. In the control group no changes occurred in any parameter of glucose metabolism. The results suggest that physical exercise is beneficial for the glycemic control in patients with mild and moderate non-insulin-dependent diabetes. Increased insulin response is at least one main mechanism of the action of exercise.