The effects of murine cytomegalovirus (MCMV) on the prenatal development of the CD-1 mouse were investigated. Two sets of experiments were performed. In the first, mice were inoculated with different doses of MCMV on gestational day 7, and in the second, pregnant animals were inoculated with a subacute injection dose at different gestational stages. The effects of maternal infection on pregnancy in terms of maternal sickness, embryo lethality, date of parturition, litter size, postnatal death, and pups' body weight on day 1 and day 3 postpartum were investigated. Two gestational stages were identified as being especially susceptible to MCMV infection. Most embryonic death as indicated by resorption rates was found after treatment on day 9, whereas perinatal death was most frequent when treatment was done on day 13 of gestation. The investigation suggests that the effects of MCMV on the CD-1 mouse vary greatly with the age of the embryo and the course of the infection.