The results of morphological examinations of 78 heart tumors in BD IX rats induced by methyl- and ethylnitrosourea (MNU, ENU) administered by various routes are described. After repeated intravenous or intraperitoneal injections of MNU heart tumors developed in 39--46% of the treated animals. The following localizations were found: left ventricle--55, right ventricle--7, both ventricles--12, other sites--4. In 51 (65.4%) of 78 cases there were early stages of tumor with a ribbon-like lining of the affected ventricle wall. They consisted of elongated stream-like-arranged tumor cells. Large tumor nodules protruding into the ventricular lumen usually showed considerable features of cellular and nuclear polymorphism. The heart tumors were classified as fibromas or neurinoma-like tumors, fibrosarcomas, and polymorphous sarcomas. Histogenetically, the neoplastic cells are probably derived from fibroblasts and Schwann cells.