We performed the comparative study of age-related changes in some physiological parameters of the muskrat, which can perform dives and therefore well adapted to the lack of oxygen, and in the non-diving laboratory rat. Analysis revealed the considerable interspecific differences in the antioxidant system and the energy supply level in the heart and kidney tissues. The age-related changes in the studied values were different between species. Results showed the unbalance in the activities of superoxide dismutase (SOD) and catalase, and also compensatory change in the level of such nonenzymatic antioxidant as vitamin E in rats during aging. The muskrat had marked age-related increase of the antioxidant enzymes activities in heart and kidney. Young muskrat of the second generation had a lower specific SOD and catalase activities compared with the first generation animals, possessed the same values as the adults. We identified the interspecific differences in the differential blood leukocyte counts. During ontogenesis the size of erythrocytes and leukocytes increase in rat, but decrease in muskrat, that is necessary for improvement of blood rheological properties and appears as an adaptation to semi-aquatic lifestyle. Age-related changes in the tissue isozyme spectrum of lactate dehydrogenase, in tissue concentration of vitamin E and of leukograms were expressed in muskrats to the lesser extent than in laboratory rats.