The encephalomyocarditis virus readily produces myocarditis in mice. Early histopathological changes in the myocardium of both newborn and adult mice are described and discussed. The histological changes noted in the myocardium conform with the presence of EMC virus crystals noted in the myocardial fibres by electron microscopy. These findings suggest that the myocardial damage is produced by direct invasion by the virus which enters the cells during infection. The relationship of EMC viral infection to cardiac disease in man is yet to be determined, but mice provide an excellent model for studying the pathology and natural history of viral myocarditis and other lesions of the heart.