SummaryFive pregnant mares, at between 6 and 8 months gestation, were experimentally infected with theBucyrusstrain of equine arteritis virus (EAV). Of the five mares, four aborted and one died. The pathogenesis of the abortions was studied, using histopathologic techniques, tissue immunofluorescence and virus isolation. Common microscopic lesions in the maternal reproductive organs indicated myometritis with a degeneration of the myocytes and an infiltration of the mononuclear cells. Epithelial cells of the endometrial gland showed sporadic degeneration. Lesions in the fetal tissue included an atrophy of the lymphoid follicles in the spleen and lymph nodes with degenerated lymphocytes. The placentae were oedematous and degenerated fibroblasts were observed in the subvillous layers. Immunofluorescence detected EAV antigen in the myometrium and the endometrial gland in the dams, in the subvillous layer of the placentae, and in the aborted fetuses. EAV was recovered from the maternal uteri, placentae and fetuses. The placentae yielded the greatest amounts of the virus. Transplacental infection of the fetus was clearly demonstrated in the EAV infection.