Exposure of mice to 1000 ppm of vinyl chloride (VC), 6 hr/day, 5 days/week, caused some acute deaths with toxic hepatitis and marked tubular necrosis of the renal cortex. Starting the sixth month, mice exposed to 1000, 250, or 50 ppm of VC became lethargic, lost weight quickly, and died. Only a few mice exposed to 50 ppm survived for 12 months. Pulmonary macrophage count was elevated in some mice. There was a high incidence of bronchiolo-alveolar adenoma, mammary gland tumors including ductular adenocarcinoma, squamous and anaplastic cell carcinomas with metastasis to the lung, and hemangiosarcoma in the liver, and, to a lesser extent, in some other organs. The incidence of these tumors quickly increased, and the severity was in direct proportion to the levels of VC and the length of exposure. Malignant lymphoma involving various organs was observed in a few mice. Rats were more resistant to the toxic effects of VC. Exposure to 1000 ppm slightly depressed the body weight of the females. Exposures of 250 or 1000 ppm caused a number of deaths and hemangiosarcoma in the liver starting the ninth month. Most rats with hepatic hemangiosarcoma also developed hemangiosarcoma in the lung. Hemangiosarcoma occasionally occurred in other tissues of one or two rats exposed to 50 ppm or higher level of VC. Exposure of mice to 55 ppm of vinylidene chloride (VDC) also caused a few acute deaths and a few hepatic hemangiosarcomas. Inflammatory, degenerative, and mitotic changes occurred in the liver. No mouse exposed to VDC developed any mammary gland tumors. Several mice had bronchioloalveolar adenoma. Exposure of rats to 55 ppm of VDC slightly depressed the body weight. Hemangiosarcoma occurred in the mesenteric lymph node or subcutaneous tissue in two rats.