The distribution of viral antigen, histological lesions and inflammatory responses were examined in brains from ovine fetuses following experimental transplacental infection with a cytopathogenic strain of bovine virus diarrhoea virus (BVDV). At 10 and 14 days post inoculation (p.i.) viral antigen-containing cells were found throughout the different zones of the developing telencephalon and cerebellum. Corresponding to the distribution of viral antigen, necrotic lesions both of already differentiated and of undifferentiated fetal brain cells occurred. The extent and severity of microscopic lesions correlated positively with the number of viral antigen-containing cells. The destructive lesions were accompanied by meningeal and parenchymal cellular infiltrations predominantly with phagocytosing macrophages. In fetuses examined at 21 days p.i. a massive necrosis of the cerebral hemispheres and severe infiltrations with macrophages and CD3-positive lymphocytes had developed. In fetuses studied between 32 and 80 days p.i. porencephaly, hydranencephaly and leukoencephalomalacic lesions were present. In brain tissue of these fetuses, with the exception of two cases, BVD viral antigen was no longer detected.