Lesions of hamster fetal neuraxial tissues, characterized by multifocal and coalescent zones of hemorrhage, edema, and necrosis in the cerebral mantle, brainstem, and spinal cord, were observed in experiments designed to test the teratogenicity of potato preparations. Retrospective and prospective data indicated, however, that the potato preparations were not responsible but that the disease occurred spontaneously in the colony and was associated with direct breeding contact of virgin females with certain males. Observations suggest that an infectious agent may be responsible, but no agent was recovered. Immunofluorescence assay of inoculated cultures indicated that reovirus was not present in affected fetal tissues.