In chronic experiments on moose with a rumen fistula, which were performed under conditions of the Pechoro-Ilychskii National Park at different seasons, it was established that in autumn as compared to summer, at a reduction of consumption of digestible nutrients and energy (more than 2-fold), the water balance decreased twofold; in the rumen, the acetic fermentation increased, while in blood, the level of volatile fatty acids (by 117% ), whereas the concentration of blood glucose and acetone bodies decreased (by 45 and 61%, respectively). It is suggested that adaptation of metabolism and energy in moose to the onset of the cold season includes a decrease of the water balance and, as a consequence, a restriction of heat losses, inhibition of aerobic oxidation of substrates, and intensification of anaerobic pathway of the organism energy supply.