Severe enteritis was produced in recently weaned, 8-week old pups 3 to 9 days after being given parvovirus by mouth. The most severe manifestations of disease were observed 7 days after infection, when one pup died. Viraemia was detected on days 4 and 5 and a severe leucopenia was present 6 to 8 days after infection. Antibody was demonstrated in serum 4 days after infection, high titres being present 3 days later. Sequential pathological studies revealed necrosis of Peyer's patches on day 3. Between days 5 and 7 typical lesions of the disease became widespread with necrosis of tonsil and thymus being prominent. By the fifth day after infection viral inclusion bodies were numerous. Virus isolation from tissues was greatest at this stage and had diminished by the seventh day. Although tissue repair was well advanced by the tenth day thymic necrosis remained prominent and villous atrophy was still present on day 13. Based on these findings a possible pathogenesis is discussed.