Abstract Estimates have been made of the numbers of functioning motor units in the extensor digitorum brevis muscles of 10 patients with myasthenia gravis. In 5 patients the numbers were abnormally low, indicating that either “pathological” or “functional” denervation had occurred. In denervated muscles the amplitudes of the potentials developed by the surviving motor units suggested that some motoneurones had undertaken collateral reinnervation of denervated muscle fibres. This selectivity of motoneurone involvement suggested the presence of a “central” lesion (motoneurone or axon) in myasthenia gravis rather than random involvement of neuromuscular junctions. In some myasthenic muscles the isometric twitches were slowed.