The greater energy requirements that accompany exercise necessitate an increase in fuel mobilization from sites of storage and a subsequent increase in the oxidation of thesefuels within the working muscle. A precise endocrine response controls the needed increment in fuel metabolism. The importance of the endocrine system is readily apparent in individuals with insulin-dependent diabetes mellitus (IDDM) in whom the normal endocrine response to exercise is lost. When a person with IDDM exercises with too little insulin in the circulation, an excessive counterregulatory hormone response may ensue, and the alreadyelevated blood glucose and ketone body levels can become even greater. On the other hand,if too much insulin is present, the exercise-induced increase in substrate mobilization can be attenuated or completely blocked and, as a result, hypoglycemia may ensue. Insulin therapy must be modified in anticipation of exercise to avoid states of under- or overinsulinization, and/or carbohydrate ingestion must be adjusted to compensate for the potential glycemic effects of inappropriate insulin levels. The presence of advanced diabetic complications can further accentuate the difficulty of exercise for those with IDDM. The purpose of this review is to provide the scientific information necessary to assess therationale for recommending regular exercise to individuals with IDDM and to develop guidelines for integrating daily exercise with insulin treatment and dietary management.