Experiments carried out in different seasons with different strains of rabies virus have shown that hibernation prolonged the incubation period in sousliks (Citellus major Pall.). Mean survival of the animals infected during the active period was 32 days, while during hibernation it was 147.3 days (P less than 0.05), in some cases up to 251 days. The virus was isolated from and/or detected by fluorescent antibody test in 50% of cases in the brown fat and in some cases in the salivary glands and visceral organs. The brown fat biopsy allowed to detect the virus not earlier than 6 days before death. We propose to use heterothermal rodents for the study of mechanism of rabies virus persistence and suggest that these animals might represent a natural reservoir for rabies.