Electromyographic (EMG) and histological studies were carried out with the new mouse mutant myotonia (mto, autosomal recessive). Affected animals exhibited behavioral myotonia which was apparent at 2 weeks of age. EMG studies revealed myotonic discharges (prolonged repetitive discharges with recurrent variation in amplitude and frequency) in all skeletal muscles tested. These discharges were not affected by peripheral nerve section or by pancuronium bromide, a neuromuscular synapse inhibitor. Qualitative histological study revealed no evidence of skeletal muscle fiber necrosis, grouped atrophy, or inflammation compared toi sex-, age-, and weight-matched controls. Degeneration of myelinated axons in the ventrolateral funiculus of the spinal cord occurred only in older animals and does not explain the functional abnormality of muscle. Both the clinical and electrophysiological features of classical myotonia can be inherited as a single locus autosomal recessive disease in mice.