Muscle has been obtained from various members of two families who are known to carry the trait for malignant hyperpyrexia. The muscle of such carriers has been found to be abnormal histologically, histochemically and ultramicroscopically, although there was no evidence of muscle disease on clinical examination. We have found that the pattern of abnormality is different in the two families, the major difference being largely related to fibre size. Both show histochemical evidence of neurogenic atrophy with reinnervation. The electron-microscopic study confirms the presence of fibre breakdown, and reveals the presence of abnormal mitochondria and membrane changes which are in keeping with a neuropathic disorder. In most cases in our studies we have confirmed that the myopathy can be established by findings of an elevated creatine phospho-kinase level in the serum. Carriers with normal levels are also identified by family studies and it has been shown that muscle abnormalities are present in these subjects as well.