Many reports have recently concerned the problems that the hypotension with autonomic nerve blocking agents are effective to prevent shock or brain edema which are often seen in craniotomy. The author studied these effects of anesthesia from the view points of electroencephalogram and brain water content. With intraperitoneal injection of 10% Dial solution for premedication, and with C(6) or Trapon as the hypotensive agents, dogs were anesthetized by endotracheal intubation with ether. In the contrast group with only Dial and ether, the increase of delta index in E.E.G. as well as that of the free and total water were seen with the lapse of time after craniotomy. In the group of hypotension (70~80 mm Hg) only with C(6) or Trapon, the increase of delta index was not seen and the brain water content showed no change. In the group of hypotension with C(6) or Trapon combined with ether anesthesia, slight decrease of delta index was seen and the brain water content showed no change. From these results, it is proved that anesthesia with simple barbiturate or ether causes anoxia of the nerve cells, resulting in brain edema and that hypotension is effective to prevent it.