The mutagenic activity of vinyl chloride (VC) at three exposure levels was assessed in fertile male CD-1 mice with the dominant lethal test. Male mice were exposured by inhalation to VC at 3000, 10,000 and 30,000 ppm for 6 h a day for 5 days. By comparison with control males exposed to air, no mutagenic effects on any maturation stage of spermatogeneisis in treated males were detected. There was no significant increase in the number of post-implantational early foetal deaths as shown by the number of females with one or more early deaths or number of early deaths/pregnancy or the number of early deaths/total implants/pregnancy. There was no evidence of pre-implantational egg losses as indicated by the total implants/pregnant female. There was also no reduction in fertility. The lack of effect was not due to the insensitivity of the system used since a dominant lethal effect was clearly demonstated in male mice dosed i.p. with cyclophosphamide (CTX) at 200 mg/kg body weight and ethyl methanesulphonate (EMS) orally at 200 mg/kg body weight once a day for 5 days. During dosing these animals were housed under dimilar exposure conditions to those animals exposed to the test substances but with a flow of air through the exposure chambers. Thus vinyl chloride is not mutagenic in the mouse at the stated exposure levels as measured by the dominant lethal test.