The activity of hexokinase and its isoenzymic spectrum were determined in skeletal muscles of different metabolic types and in brain of trained and untrained albino rats. It is shown that adaptation to intensive muscular activity accompanied by metabolic acidosis induces some changes in the hexokinase system, which consist in the increase of the total hexokinase activity not only under the optimal conditions but also with more acidic pH values of the medium and in the rise of the quantity of the muscular type isoenzyme which is more stable to the pH action.