Prolonged administration of dried Senecio longilobus in the diet of rats led to necrosis and regeneration of liver cells with hypertrophy of some cells and nuclei, bile duct proliferation, and neoplasia of parenchymal cells. Additional changes included pulmonary arteritis and pulmonary adenomatosis. The highest tumor yield was obtained by alternate weekly feeding for 1 year of a diet containing 0.5% Senecio and a Senecio-free diet. Liver tumors developed in 22 male and 6 female rats. In 14 rats, sections showed invasion of contiguous veins. In 7 rats there were 2 distinct primary tumors. In 6 rats, there were intrahepatic metastases; 3 of these also showed pulmonary metastases, and several branches of the pulmonary arteries of another were plugged by proliferating tumor cells. Two rats had hepatic angiosarcomas with multiple nodules.