The purpose of this study is to delineate the histopathologic findings of the spleen after Hantaan viral inoculation, which is the largest lymphoid organ in rats, and to identify the viral location by anti-Hantaan virus (HTNV) monoclonal antibody. All the sixty one suckling rats of less than twenty four hours of age were used. Except twenty one rats of control group, twenty-five rats inoculated intracerebrally for the early change and fifteen suckling rats inoculated intramuscularly for the late change were uniformly susceptible to lethal infection with the ROK 84-105-1 strain of seed HTNV. The characteristic histopathologic findings were; appearance of macrophages below the splenic capsule on the 3rd day, small lymphocytes around the periarteriolar sheath on the 5th day increasing in numbers on the 7th day, and a markedly expanded marginal zone with some immunoblasts and plasma cells as well as decreased extramedullary hematopoiesis on the 9th and 14th days. Time of onset of histopathologic changes in spleen thickness, appearance of medium and large lymphocytes and degree of extramedullary hematopoiesis were influenced by inoculation route, whereas expansion of the marginal zone was affected by postnatal age.