Pregnant C3H mice were orally treated with 50 mg Trypaflavin/kg on day 7, 11, 14, or 15 post conception. The embryos were thus treated in utero with the test compound. At the age of 10 weeks, the dominant lethal assay was performed with F1 females. Dominant lethal mutations were induced only in those mice treated in utero on day 7 of the prenatal stage. Female C3H mice were chronically treated with Trypaflavin (50 X 2 mg/kg/day; dissolved in drinking water). These mice were caged with untreated males. The percentage of preimplantation egg loss and the yield of dead implants per female was increased. Female NMRI mice were chronically treated with Trypaflavin (50 X 2 mg/kg/day by stomach tube). In metaphases II of unfertilized oocytes, the yield of all observed aberration types (aneuploidies, gaps, satellite associations, breaks and fragments, deletions, and interchanges) was increased weakly.