In 13 Baikal seals Pusa sibirica (the body mass of 11–73 kg) on the empty stomach and at free swimming in a reservoir, gas exchange parameters were studied. In 5 of these individuals, the gas exchange was assessed in satiated and hungry animals. The numerical correlations are presented between the oxygen requirement of these both groups of animals and their body mass. The data obtained show that the basal metabolism of seals corresponds to that of terrestrial mammals and in the seals with the body mass of 40 kg and more, the temperature of the lake habitat is close to thermoneutral. The specific dynamic effect of food (fish) in the seals in the thermoneutral zone is 22–93%.