Many discussions have been made about the mechanism or the cause of brain edema. The writer planned to study the difference among various kinds of brain edema with different causes, experimentally by measuring the water content of cats' brain. In the cases of brain edema caused by ligation of the bilateral jugular veins, injection of destilled water into the carotid artery and experimental commotio cerebri, the free as well as the total water increased and the bound water showed no change, thus the "edematous type" resulted. The increase of the free as well as the total water was most striking in those with injection of destilled water into the carotid artery, then in those with ligation of the jugular veins and less in those with commotio cerebri. In the cases of brain edema caused by irrigation of warm physiologic saline solution upon the surface of brain, the free as well as the total water decreased and the bound water increased, thus the "swelling type" was seen. In the cases with experimental chronic internal hydrocephalus, no change was seen in the total, free or bound water.