Two groups of puppies, eight and 10 weeks of age, were inoculated orally with canine parvovirus of faecal origin. The patterns of faecal excretion of virus, antibody production and systemic viral localisation following inoculation were studied. Faecal excretion of virus was first apparent at day 3 after inoculation, was present most frequently and in greatest quantity at days 4 to 7 after inoculation and fell sharply thereafter. Serum antibody was first detected at day 5 after inoculation with high titres in all samples from day 7 onwards. Virus isolation from serum samples revealed a non-cell associated viraemia at days 3 and 4 after inoculation. Immunocytochemical examination, using both immunofluorescence and immunoperoxidase techniques, first revealed antigen in the thymic cortex at day 1 after inoculation and in the germinal centres of the lymph nodes and the splenic white pulp from days 2 and 3. Viral antigen was first detected in the intestines at day 4 in individual cells in the proliferative zone of the crypt epithelium. From day 5 onwards, the amount of antigen present in the lymphoid tissue decreased so that by days 7 and 8, only a trace was present. There was widespread specific staining in the small intestinal mucosa at day 6, but little antigen was present by day 7. Virus was present in the bone marrow of some dogs killed at days 5 and 6.