The muscle physiology and histochemistry of a hereditary neuromuscular syndrome of the mouse, “arrested development of righting response” (ADR), was studied. The speed of single twitches of fast ADR limb muscles was normal up to an age of about 60 days but decreased at later ages. At any age between 10 and 120 days postnatal, fast and slow muscles of the mutant displayed after‐contractions of 1–3 (5) seconds duration. These coincided with electrical after‐activity of muscle, as demonstrated by electromyography, After‐contractions and EMG signals were suppressed by the membrane‐stabilizing drug tocainide. These physiological data suggest that ADR is a myotonia. With a few exceptions, limb and trunk muscles of ADR animals showed a uniform oxidative phenotype with a lack of large diameter glycolytic fibers. The histochemical muscle phenotype of the ADR mouse was partially reversed by a long‐term treatment with tocainide.