In 7 men and 1 woman who died suddenly the functionally important areas of myocardium in the sino-auricular area and the subendocardial layers of left ventricle were obtained by necropsy no more than 3 h after death, and then prepared for study by electron microscopy. In three cases of five, in whom the cause of death was cardiovascular insufficiency, viral particles and bacteria were identified. In three other cases of sudden non-cardiac death they were not found. Viruses were found only in working cardiomyocytes, but near the sinus node. Bacteria were found in left ventricular subendocardium. Intercellular junctions between virus-damaged myocytes and intact cells were preserved. Based upon the functionally important sites where they were found, associated degeneration near them, and preservation of contacts among myocardial cells there, viruses and bacterial infections may play an important role in the patho-physiologic events leading to some cases of sudden death.