Genital mycoplasmosis is important as an animal model for the interaction between infectious agents and the host during pregnancy as well as in its own right as a confounding variable affecting research projects in which the rat is used as a model to study reproductive function and physiology. We report the in utero transmission of Mycoplasma pulmonis and the development of placentitis, amnionitis, and mild fetal bronchopneumonia in Sprague-Dawley rats. A minimum of 10 days prior to breeding, specific-pathogen-free female Sprague-Dawley rats were infected by intravaginal inoculation with 3 x 10(7) CFU of M. pulmonis X1048 or with an equal volume of sterile broth. Rats and fetuses were subjected to necropsy at days 11, 14, and 18 of gestation. M. pulmonis was able to invade the placenta, cross the placental barrier, and establish an amniotic fluid infection by gestational day 14. It was isolated from the oropharynx and lungs of fetuses at gestational day 18. The placenta was more frequently colonized than amniotic fluid, followed by the fetal oropharynx and lungs, supporting an ascending route of infection. Histopathological evidence also support an active infection, with lesions compatible with placentitis, amnionitis, and mild fetal bronchopneumonia. M. pulmonis can traverse the placenta, resulting in infection of the amniotic fluid and in utero transmission of the microorganism to the developing fetus.