Ultrastructural immunocytochemical studies of the expression of major histocompatibility complex class I antigens in the placentas of inbred rats were performed using placentas derived from natural matings and from embryo transfers into females made pseudopregnant by mechanical stimulation. The studies utilized the WF (u) and DA (a) strains and monoclonal antibodies to all of the class I antigens involved. All four mating combinations of the two strains showed that only paternal antigens were expressed in the placenta and that they were limited to the basal trophoblast. This conclusion was confirmed using embryo transfer experiments. In allogeneic natural matings, the allele-specific class I transplantation antigens were not expressed on the membrane of the basal trophoblast but they were expressed in embryo transfers involving embryos of the same genotype. In both types of pregnancies, the pregnancy-associated (Pa) antigen was present on the membrane of the basal trophoblast. The antibody response to DA X DA and to WF X DA embryos transferred into pseudopregnant WF females was against the allele-specific RT1.Aa antigen and the Pa antigen, whereas the antibody response to the heterozygous embryo in the natural WF (female) X DA (male) mating was against the Pa antigen only. These results are consistent with the hypothesis that the suppression of the expression of the allele-specific major histocompatibility complex class I antigens occurs shortly after fertilization and that it requires the uterine environment of a natural mating.