The occurrence of Hemorrhagic fever with renal syndrome was noted for the first time among the United Nations troops in Korea in 1951. In 1976, Lee and Lee demonstrated for the first time an antigen in the lungs of a striped field mouse, Apodemus agrarius, which gave specific immunofluorescent reactions with sera from patients convalescent from Korean hemorrhagic feve (KHF). The natural reservoir host of KHF is Apodemus agrarius coreae mice in rural endemic areas in Korea. Clinical manifestations of acute illness do not occur in infected Apodemus agrarius. Infected rodents excrete large amounts of virus in saliva, in urine and in feces for a long period of time. In this study, the tissues of Apodemus agrarius, Wistar rat and Balb/C mouse were inoculated with Hantaan virus and subsequently observed for any histopathologic findings. 1. In lungs, infiltration of lymphocytes in alveolar septa and peribronchial region and thickening of and giant cell formation in alveolar septa were observed. 2. In kidney, vascular congestion, interstitial hemorrhage in corticomedullary junction and partial microscopic hemorrhage in urinary space were found. 3. In spleens, vascular congestion, old and recent hemorrhage and multinucleated giant cell formation were seen.