The EEG patterns of increasing depths of Ēthrane-O2 anesthesia and the alterations in the patterns produced by changing Paco2 and by addition of 60 per cent nitrous oxide have been evaluated in 30 individuals. Increasing depth of anesthesia was characterized by the appearance of high-voltage spikes, with the subsequent development of spike waves and burst suppression. Maximum depth was marked by a predominance of spike waves and burst suppression. The anesthetic depth and Paco2 at which changes occurred have been defined. With elevation of Paco2, indices of cerebral irritability appeared to be reduced, while reduction of Paco2 increased their occurrence. The greatest CO2 effect was seen at inspired Ethrane concentrations of 2.5 per cent or more. The addition of nitrous oxide did not alter the predominant EEG patterns.