This review describes (1) the metabolic and hormonal response to exercise in normal and diabetic man, and (2) the potential benefits of physical training in diabetes. Whereas in normal man plasma glucose varies little during exercise, the insulin-dependent diabetic subject may experience an increase in plasma glucose, a modest decrease or a marked decrease which can result in symptomatic hypoglycemia. Evidence is reviewed that the glycemic response depends on the ambient plasma concentration of insulin and that this may be influenced by an effect of exercise on the absorbtion of insulin from its site of injection. The response to exercise of noninsulin-dependent diabetic subjects and of diabetic subjects with autonomic neuropathy is also described. Physical training improves glucose tolerance in some noninsulin-dependent diabetic subjects and in insulin-dependent patients, it may diminish insulin requirements. It may also have a role in retarding the development of cardiovascular complications. Physical training is not totally innocuous, however, and in many patients with diabetes special precautions are required.