To establish reliable criteria for the depth of fluothane anesthesia in dogs, an electroencephalogram (EEG) and several physiological responses were recorded simultaneously. EEG was recorded from four points on the cerebral dura mater. Respiration, electrocardiogram (ECG), electrooculogram, electromyogram (EMG) in the neck muscle, and blood pressure were recorded simultaneously together with the EEG. Anesthesia was induced with 4% fluothane in a closed system. Six distinct levels were defined as follows. Level I: This level was the induction stage. The resting fast-frequency and low-amplitude rhythm was replaced by a rhythm slower in frequency (15 to 30 Hz) and higher in amplitude (20 to 70μV). Level II: The characteristic of this level was the appearance of spindling burst waves with a frequency of 13 to 25 Hz. Level III: This level was the period of light anesthesia. Predominant waves showed a frequency of 5 to 13 Hz and an amplitude of 50 to 200μV. Level IV: The predominant waves decreased in frequency. All the fast waves disappeared completely. Generally, EEG waves had a tendency to flatten. This level corresponded to the period of deep anesthesia. Level V: The frequency of slow waves was 2 to 7 Hz. The amplitude of waves tended to be low. Sometimes EEG showed a short burst suppression. Level VI: This level was the period of long apnea. EEG showed a flat pattern. From the results of the present work, it was concluded that the changes of EEG were closely related to the depth of anesthesia and physiological changes in dogs anesthetized with fluothane.