To study the development of lesions in mucosal disease, the spread of cytopathogenic (cp) bovine virus diarrhea virus (BVDV) to different organs was examined in relation to the time post inoculation (pi). Mucosal disease was induced in 15 persistently viremic cattle from two herds by intranasal inoculation with antigenically similar cp BVDV strain. This strain reacted with one additional monoclonal antibody when compared to the corresponding herd-specific non cytopathogenic (ncp) isolate. Twelve cattle were euthanized at days 3, 5, 7, 9 and 13 pi in the early phase before they developed clinical signs of mucosal disease, three in the advanced phase when they were moribund and three served as controls. Antigen of the cp BVDV strains was selectively detected in tissue sections by immunohistochemistry. In the early phase, varying amounts cp BVDV were present most consistently in tonsils, lymph nodes, Peyer's patches and lymphoid nodules in the large intestine. In the lymphoid tissues, first a few cells in single lymphoid follicles, then groups of lymphoid follicles contained antigen. In intestinal epithelium, cp BVDV antigen was found focally in the early phase of mucosal disease. Its diffuse distribution in the late phase corresponded with clinical signs of diarrhea.