Summary Five patients with a history of post‐exercise bronchoconstriction and eleven control subjects were exposed to gradually increasing work loads on a bicyle ergometer. The asthmatic patients showed higher blood lactic acid levels at all work loads than the control subjects. In contrast to findings in the controls, the plasma free fatty acid in the asthmatics failed to ‘rebound’ following the cessation of exercise; and in two out of three patients plasma FFA did not rise after epinephrine injection. In addition, the asthmatic patients consumed less oxygen during the exercise than the controls. The results of this study suggest that patients with post‐exercise asthma may have to rely more on carbohydrates as the main source of energy because of the reduced availability of FFA. Since the consumption of oxygen is also reduced, this may lead to a higher lactate production.