It is known that the hypothermic anesthesia with autonomic nerve blocking agents causes not only less bleeding, but also prevents shock or brain edema which is often seen in craniotomy. The author studied these effects of potentiated narkosis or hypothermia from the view point of electroencephalogram and brain water content. With intraperitoneal injection of 10% Dial solution for premedication in all cases and with chlorpromazine as the blocking agent, dogs were anesthetized by intratracheal intubation with ether, and were cooled by ice bags until the rectal temperature became about 34°~32℃. And the following results were obtained. In the group of potentiated narkosis with Dial and chlorpromazine, the decrease of delta index was seen and the brain water contents showed no change compared with the contrast group. In the group of hypothermia with chlorpromazine, the decrease of delta index was seen and the brain water content showed no change. In the group of hypothermic anesthesia with chlorpromazine and ether, the result was the same as the former group. By this study it was explained that these effects are not only due to less dose of anesthetic agents in combination with autonomic nerve blocking agents, but also to the rare occurrence of the cerebral anoxia, and that the hypothermia is effective to prevent brain edema in craniotomy. It is also considered that the oxygen inhalation during and after the operation may preveit the so-called brain swelling.