This study examined the behavioral and physiological effects of long-term exposure to overdose of aversive odor (predator odor) in two species of hamsters. About 0.05 mg of anal gland secretions of Siberian weasels (Mustela sibirica) was smeared at the oronasal groove of wild male ratlike hamsters (Cricetulus triton) (natural prey) and laboratory golden hamsters (Mesocricetus auratus) once every day for 4 weeks. After 28 days, the experimental groups of both hamster species displayed higher cortisol level, larger adrenal gland (in ratlike hamsters only), smaller thymus and flank gland, and lower aggression level than the conspecific control group (presented with water). Thus, the long-term presence of overdose of the anal gland secretion of the Siberian weasel could lower the aggression and social rank and suppress the immunity in the hamsters. The reproductive conditions of these prey species, however, seemed not to be affected. In addition, the similarities in the behavioral and physiological responses to the predator odor between the two species of hamsters showed that the responses to predator odor might be innate.