The herbicides paraquat and diquat were tested for dominant lethal activity in male CD-1 mice. No mutagenic effects were detected on any stage of spermatogenesis in treated males if these compounds were administered orally up to 4 mg paraquat/kg/body weight/day of 10 mg diquat/kg/body weight/day for 5 days. There was no increase in the number of post-implantation losses 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/pragnancy. There was no evidence of pre-implantation losses as indicated by the total implants/pregnant female. There was no anti-fertility effect on the treated males as measured by the pregnancy frequency or successful mating frequency. The lack of dominant lethal effect was not due to particular insensitivity of the animals used since such effects were amply demonstrated in mice doses orally on 5 days with ethyl methanesulphonate at 100 mg/kg/body weight/day, or intraperitoneally on one day with cyclophosphamide at 200 mg/kg/body weight.