The preliminary results of a retrospective examination of the myocardium from 11 patients who responded to general anaesthesia with malignant hyperthermia are presented. Light-microscopical examination of the sections revealed no specific changes due to malignant hyperthermia. Contraction bands were demonstrated in 4 cases and it is concluded that the present investigation does not support the suggestion that a specific myofibrillar damage is responsbile for the arrhythmias seen in malignant hyperthermia.