A study of the sex ratio in mice of inbred strains CBA and C3H and of the connection between the postimplantation embryonic mortality of mice of these strains and the sex distribution of the embryos showed that the embryonic sex ratio of these lines of mice obeys the 1∶1 distribution. Data in the literature and the writer's own observations suggest that genetic differences between mice of inbred strains have no significant effect on the sex ratio of the progeny. The postimplantation embryonic mortality in C3H mice is greater than that of CBA mice (14.4 and 9.3% respectively). However, the presence of a balanced sex ratio in the mice of these strains is evidence of absence of selective death of embryos of either sex during embryogenesis.