This study was designed to test the hypothesis that there is a myopathic component in myasthenia gravis. Serum enzyme analyses and lactate and pyruvate assays were undertaken before and after exercise in order to detect possible leakage from defective muscle membrane. A patient and a control group were exercised under (1) aerobic conditions, (2) ischaemic conditions, and (3) aerobic conditions under the influence of edrophonium chloride. The patient group showed a higher resting level of lactate in the aerobic state than did the controls. In addition, the average change in SGOT values in the patient group after one minute of aerobic exercise under the influence of edrophonium chloride exceeded that of the control group. Both these findings suggest possible leakage of enzyme and metabolite from the myasthenic muscle due to a membrane defect and support the hypothesis that there may be a myopathic component in myasthenia gravis. However, the two groups showed no significant differences in CPK values, which are often elevated in primary muscle diseases.