ABSTRACT. Puumala (PUU) virus and PUU-related viruses are difficult to isolate in cell culture. To determine whether animal inoculation would be a better alternative for virus recovery, the Sotkamo strain of PUU virus was inoculated into several animal species. Newborn Mongolian gerbils (MGs), mice, and rats were infected with the Sotkamo strain by intracerebral (ic), intraperitoneal (ip), and subcutaneous (sc) inoculation. Antibodies to PUU appeared in MGs at 30 days post-infection (dpi), and in mice and rats at 15 dpi. Interestingly, virus appeared at 7 dpi in lung and brain of MGs inoculated via ic and ip routes. Virus was detected in all tested tissues of MGs at 15 dpi, with a peak level of 1.36 × 10 5 focus forming units (FFU)/g in brain tissue. The virus titer declined with the onset of the antibody response and became undetectable by 75 dpi, when the antibody titer reached the maximum level. The appearance of the virus in mice and rats was delayed as compared to MGs, and the virus titer was apparently lower, at approximately 4 to 8 × 10 3 FFU/g, at 15 dpi. In addition, lung homogenates of antibody-positive Clethrionomys (C.) rufocanus (captured in Tobetsu, Hokkaido, Japan) were inoculated into MGs by the ic route. PUU-related viral RNA was detected at 16 dpi in the brains of MG inoculated with the lung homogenate, and antibodies were detected at 45 dpi. These findings indicate that newborn MG inoculation is an efficient method to recover PUU and PUU-related viruses. KEY WORDS: Clethrionomys rufocanus, Hantavirus, Mongolian gerbil, Puumala virus, Rodent.J. Vet. Med. Sci. 65(11): 1189-1194, 2003 Hantaviruses cause hemorrhagic fever with renal syndrome (HFRS) in Europe and Asia, and hantavirus pulmonary syndrome (HPS) in the Americas [6,16,22,25,26]. It is believed that there is a close relationship between hantaviruses and the specific rodent species in which they establish inapparent and persistant infections [23]. Hantaan (HTN), Seoul (SEO), and Dobrava (DOB) viruses cause a severe form of HFRS and are carried by Murinae rodents [1,2,17,18]. Arvicolinae rodents are considered to be the reservoir hosts of Puumala (PUU), Tula (TUL), Topografov (TOP) and Khabarovsk (KBR) viruses [15,23]. Sigmodontine rodents are the most common rodent species in the Americas and are the reservoir animals for Sin Nombre (SN), Andes (AND), Bayou (BAY), Black Creek Canal (BCC), and New York (NY) viruses [7,10,11,19,20,24,27]. Primary hosts shed viruses into urine, saliva, and feces, and virus transmission to humans occurs via aerosolized rodent excreta [21,28] or via contaminated saliva in animal bites [8].Members of the genus Hantavirus of the family Bunyaviridae are negative strand RNA viruses with tripartite genomes. The segments have been designated, based on their sizes, as large (L), medium (M), and small (S). The L segment encodes a viral RNA polymerase, while M and S encode two surface glycoproteins G1 and G2, and a nucleocapsid protein (NP), respectively [9].PUU virus is the causative agent of a mild form of hemorrhagi...