Blood glucose, lactate, insulin, C-peptide, norepinephrine and epinephrine concentrations were determined in non-insulin-dependent diabetic patients and in healthy controls before, during and after moderate exercise, to evaluate the effects of physical exercise on glucoregulation. Ten diabetic and ten healthy control females bicycled 14 minutes at 60% of their maximal heart rates. In the diabetic patients, there were no significant changes in blood glucose levels post-exercise, while in controls the 60 minute post-exercise levels were higher than those measured in mid-exercise (p < 0.05). Lactate concentrations increased with exercise in both groups in a similar manner, with highest values at the end of exercise. No significant changes in insulin and C-peptide levels were induced with exercise in either group. Norepinephrine and epinephrine concentrations increased 2.5-3 fold with exercise in both groups (p < 0.05 for all values) but in the diabetics an earlier and prolonged catecholamine response was observed. We propose that catecholamines prevent hypoglycaemia during exercise when changes in insulin and C-peptide do not occur. In diabetic patients with good metabolic control, the glucoregulatory response to exercise is not worse than in anthropometrically similar controls with similar levels of fitness.