The solutions of hydrochloric acid and natrium bicarbonicum were given to rabbits intravenously. The reserve alkali was measured by VAN SLYKE method, and the water content was measured by HATSCHEK' cobalt chloride method. The results were as follows; The brain water content increased rapidly at the point of 6-7% increase of the reserve alkali. It showed also considerable increase already at the point of 2.3% decreaae of it. It increased a little more by the further change of reserve alkali beyond these limits, but was almost fixed. This increase of brain water content depended on only the free water and the bound water was constant. By this study it was proved that, the increase of brain water content depended on not only the increase of blood caused by dilatation of brain vessels, but the increase of water conent in brain itself, and that the oxygen inhalation was very effective to prevent the so-called brain swelling during and after the operation.