Six EEG levels of Ethrane anesthesia, similar to those of ether, were observed in cats with chronically implanted recording electrodes. With the onset of surgical anesthesia (EEG level III and deeper), high-amplitude spikes and/or spike-wave-complexes invariably appeared in all leads. In three cats faint twitching of the muscles of the face and tongue synchronous with spike discharges appeared at levels III and IV. Deepening or lightening the level of anesthesia stopped the twitching. As anesthesia deepened and the duration of burst suppression lengthened, spiking became more rhythmic Bursts of single or grouped high-voltage spikes arising from a nearly flat baseline commonly were the only major ongoing electrical activity observed in deep level V and in level VI. Noise increased the frequency of spike-like discharges in two deeply anesthetized (level V) animals