The effects of congenital infection with cytomegalovirus on fetal development were studied by injection of murine cytomegalovirus (MCMV) into the endometrial lumina of mice in the early stages of pregnancy. The mice were later killed, and the sites of embryonic implantation were examined. Litter size was reduced and the incidence of abnormal fetuses was significantly increased among MCMV-infected animals. Morbid, moribund, severely growth-retarded, asynchronously developed, and malformed fetuses were found. Virus was reisolated from infected mothers, placentas, and fetal tissues but not from control animals that had received either heat-inactivated virus or culture medium. The recovered virus was identical to the original virus, as verified by restriction enzyme analysis. MCMV-specific DNA was detected in experimental tissue sections by in situ nucleic-acid hybridization. Seroconversion occurred in infected female mice. Thus, the presence of MCMV in the genital tract at the time of embryonic implantation causes not only embryonic infection but also fetal maldevelopment.