beta-Adrenoceptor antagonists influence the metabolic responses in man at rest and during exercise. Impaired working capacity and muscular fatigue have been reported in patients on beta-blockers and this could be due to an altered substrate supply to the muscles. The results from several studies show that the main effect of beta-blockade on metabolism is decreased lipolysis, with less fat available to the muscles. This results in an increased carbohydrate demand to maintain an unchanged aerobic metabolism, and liver and muscle glycogen stores are more rapidly depleted. beta-blockade also results in decreased lactate release from the muscles, probably due to a membrane effect and/or changed perfusion. It is concluded that beta-blockade a) decreases fat metabolism in the muscle, which secondarily increases the use of carbohydrates during exercise, resulting in earlier hypoglycaemia and/or depletion of muscle glycogen with reduction of the working capacity, b) impairs lactate transport from the muscle but does not cause lactate accumulation within the muscle which could be responsible for muscular fatigue.