An attempt has been made to analyse electroencephalogram according to frenguecy analysis, Fluothane concentration in blood and clinical observation of anaesthetic depth under 2% Fluothane anaesthesia. Forty-two adults in both sexes were utilized whose electroencephalograms has been revealed normal under awakness. Demerol, Scoplamine and Pentobarbital were preliminarily given and semi-closed method of anaesthesia was done with either a mase or a tube which was inserted under 2% Xylocaine block of superiorlaryngeal nerves and of larynx directly through neck. Those premedications were hardly found to affect the E. E. G., but, under Fluothane anaesthesia, gradual decrement of the freguency and increment of the amplitude were to be noted. Frequency analysis also revealed under a tendency of gradual increment from β(1) zone to δ(s) zone. It is to be noted that slow frequency zone was not necessarily to reach the basic freguency to gain the sufficient anaesthetic stage for surgery. Such E. E. G. findings were found to be in parallel with the Fluothane concentration in blood, and to correspond with the anaesthetic depth in clinical category.