Metabolic myopathies due to a variety of enzymatic deficiencies are well recognized. The dynamics of oxygen delivery and utilization during exercise have not been observed previously in these disorders. We used a noninvasive optical technique to measure oxygen consumption in the exercising limb in normal subjects and patients with metabolic myopathies. We measured near-infrared spectra of hemoglobin in the gastrocnemius muscle during treadmill exercise in 10 normal subjects, 1 patient with cytochrome c oxidase deficiency, 2 patients with myophosphorylase deficiency, 3 patients with phosphofructokinase deficiency, and 2 patients with carnitine palmityl transferase deficiency. All normal subjects demonstrated a sustained deoxygenation during exercise, indicating an efficient utilization of delivered oxygen. The patient with cytochrome c oxidase deficiency demonstrated consistent oxygenation during exercise, indicating an underutilization of delivered oxygen. In the patients with myophosphorylase or phosphofructokinase deficiency, abnormal oxygenation during exercise indicated an oxidative defect due to a lack of pyruvate production. In the patients with myophosphorylase deficiency, changes in oxidation coincident with glucose utilization and "the second wind phenomenon" were observed. Patients with carnitine palmityl transferase deficiency demonstrated a normal deoxygenation during exercise. Noninvasive tissue oximetry during exercise demonstrates specific abnormalities in a variety of metabolic myopathies, indicating abnormal oxygen utilization, and will be a useful addition to the clinical investigation of exercise intolerance.