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The derivation and maintenance of inbred strains of animals is frequently difficult or impossible due to reductions in litter size and viability of the offspring. Reductions in litter size might be expected from adverse genetic or environmental effects on components of the reproductive cycle including the ovulation rate, the rate of preimplantation mortality of the zygote and the rate of postimplantation mortality of the embryo. In some strains ofinbred mice, the reduction in litter size is largely due to postimplantation losses (Lyon, 1959; Krzanowska, 1960; McCarthy, 1965, 1967). When female mice from inbred strains were mated to males of another strain, the incidence of post- implantation mortality was significantly reduced (McCarthy, 1965). It has been suggested (Lyon, 1959; McCarthy, 1965) that this wastage is the result of an increased number of recessive lethal genes, either stabilized in the strain or recent lethal mutations undergoing segregation. The implication is that the high rate of postimplantation mortality is the result of the poor genetic con¬ stitution of the young and that the maternal contribution should have little effect on the number of early deaths. On the other hand, studies involving the mouse (Krzanowska, 1960; Bradford & Nott, 1969) and rabbit (Hammond, 1928) present some evidence that the maternal environment may be involved in determining the rate of embryonic mortality. In rats, we have observed a high rate of both preimplantation and post- implantation mortality in the inbred ACI strain. Preimplantation losses were 25-8% of the total number ofova shed in normal ACI females ( 118 implantations /159 CL) and are approximately equal to the rate ofpostimplantation mortality. To examine any effect of genotype in determining the increase in postimplanta¬ tion mortality, we conducted a series of breeding experiments utilizing the ACI and F344 inbred strains (Microbiological Associates, Bethesda, Maryland). In each mating combination, one male was caged with three females for a period of 5 days. The females were killed 12 days later, and the number of viable and non-viable implantation sites were recorded ( 12 to 17 days gestation). Urogenital abnormalities have been reported in the ACI strain (Morgan, 1953; Deringer & Heston, 1956; Fujikura, 1970) and, at the termination of the experiment, animals with abnormalities were eliminated from the data to reduce the potential influence of this defect on the mortality rate. A randomly

MATERNAL INFLUENCE ON POSTIMPLANTATION SURVIVAL IN INBRED RATS