This report summarizes the results of 9 months exposure to various levels of vinyl chloride (VC) and one level of vinylidene chloride (VDC) in rats and mice. At the end of 9 months exposure of rats to 50, 250, or 1,000 ppa VC, or 55 ppm VDC for 6 hours/day, 5 days/week, a number of adverse effects were found. Between the 33rd and 39th week, one male and three female rats exposed to VC died or were terminated. A number of tumors were found in these rats as well as in some of the rats terminated at the end of 9 months exposure to VC. Hemangiosarcomas were observed in liver, lung and adrenal gland of rats exposed to 250 or 1,000 ppm VC. The tumor incidence was higher in the rats exposed to 1,000 ppm VC than in rats exposed to 250 ppa VC. The body weight gain of female rats exposed to 1,000 pm VC was slightly depressed. Exposure to 55 ppm VDC slightly depressed the body weights of both male and female rats. At the end of 9 months exposure to VC or VDC, no adverse changes in clinical laboratory data occurred.