Maximum M‐waves (muscle compound action potentials) were studied in the abductor pollicis brevis (APB), extensor digitorum brevis (EDB), and tibialis anterior (TA) muscles of 12 patients with myotonic muscular dystrophy (MMD) and in the same number of control subjects, matched for age and sex. The peak‐to‐peak amplitudes and voltage‐time areas of the responses were measured at rest, between 40 maximum voluntary contractions (each lasting 3 s) and also during a 2‐min recovery period. In 34 of the 36 control muscles the M‐waves potentiated during the period of intermittent voluntary contractions. In the MMD patients, however, the M waves exhibited initial declines in 25 of 30 muscles. In the APB and EDB muscles the normalized mean values for the smallest M‐waves, recorded during the 350 s total observation periods, differed significantly between the 2 groups of subjects. It is suggested that the sarcolemmal Na+‐K+ pump has a raised threshold for activation in MMD patients.