In most patients with myasthenia gravis, the mean duration and amplitude of motor unit potentials sampled in the extensor digitorum communis muscle, as well as the amplitude of the pattern of activity during full effort, increased after application of anticholinesterase. This appears to be a sensitive test for early detection of failure in neuromuscular transmission. In a series of 12 patients, the mean amplitude of motor unit potentials increased 30%-130% in 9, and the mean duration increased 10%-25% in 7. In 3 patients who failed to show an increase in amplitude in the extensor digitorum communis muscle, the amplitude of motor unit potentials increased in a facial or extraocular muscle after intravenous application of edrophonium. This increase in the amplitude or duration of motor unit potentials was even evident in 4 patients who failed to show a decrement in response of the opponens pollicis muscle to repetitive stimuli delivered to the median nerve at 3/sec and 5/sec. In 5 patients, responses evoked by stimuli delivered at 15/sec showed marked facilitation.