The gas content in blood was changed by artificial pneumothorax in cats and measured by VAN SLYKE method. The water content in the cerebral motor cortex, white substance, cerebellum and brain stem was measured HATSCHEK's cobalt chloride method. The results were as follows; In normal cat brain the water content was different in each part; the most in the cerebral motor cortex, and became less in the order of cerebellum, brain stem and cerebral white substance. The water content increased rapidly when the ratio of O(2) decrase in the arterial blood became more than 18-22% and when that of CO(2) increase over8-10% And when the decrease of and O2the increase of CO(2) became more, the water content accordingly showed a further increase, but almost constant. This increase of brain water content depended upon only the free water while the bound water was always constant. There was almost similar increase in each part above mentioned.
By this study it was explained that, the increase of brain water content became rapid at the point of a certain decrease of O(2) content and increase of CO(2) in blood. This fact told us a suggestion of the genesis of brain edema and that the oxygen inhalation during and after the operation would prevent the so-called brain swelling.