Embryonic mice at 8.5 days gestation, during active neurogenesis, from pure strains (CF1 and C57) and their hybrids were X-irradiated to 200 r and examined at 18.5 days for the incidence of resorptions and anomalies. Some 302 pregnancies and 2630 implantation sites were examined. The hybrids showed a higher level of apparently normal fetuses and a lower level of resorptions than either of the parental stocks, indicating some degree of heterosis established by the time of exposure (8.5 days). However, while there was better survival and lower level of resorption in the hybrids, among the survivors there were anomalies which did not appear among the pure strains where intra-uterine resorptions killed off many of the embryos.