The effects of aging on the induction of angiosarcomas by vinyl- chloride (75014) were studied in rats. Male and female Sprague- Dawley-rats aged 6, 18, 32, and 52 weeks were exposed to 940 parts per million of vinyl-chloride by inhalation 7 hours a day, 5 days a week for 24 weeks. Matched control groups of the same ages were housed similarly but not exposed to vinyl-chloride. Autopsies were performed on all animals that died spontaneously, were killed when moribund, or were sacrificed 3, 6, or 9 months after initial exposure. The incidence of angiosarcoma increased with increasing age at first exposure. No males exposed at 6 and 18 weeks had angiosarcomas at sacrifice, but the incidence increased to 6.7 percent in the group first exposed at 32 weeks and 24 percent in the group first exposed at 52 weeks. Incidence rates for females were 5.3, 15, 47, and 20 percent for exposure at 6, 18, 32, and 52 weeks, respectively. The first angiosarcoma appeared earlier in females of each age group than in males. Most of the angiosarcomas were highly anaplastic primary tumors in the liver that metastasized to the lungs. Only one angiosarcoma occurred in control animals, and that was in subcutaneous tissue. The authors conclude that older adult animals are more susceptible to angiosarcomas caused by vinyl- chloride than younger adults and that females are more susceptible than males.