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A quantitative investigation of the EEG during hypnosis was made by analyzing the analogue power frequency spectrum of one group of subjects in the awake and hypnotized conditions, and another group (random sample) in the awake condition. Individuals of the first group were thoroughly experienced in self-hypnosis and highly hypnotizable, whereas those of the second group had never been hypnotized and were low in waking suggestibility There were no statistically significant differences in mean power of the whole EEG spectrum between the awake and hypnotized conditions of the experimental group, although a trend toward increased θ (4 to 8 Hz) density during hypnosis was apparent. This group, however, exhibited significantly more θ activity during both the hypnotized and the awake conditions than the random sample of controls in the awake condition, irrespective of whether the eyes were closed or open. We suggest that this increased θ density in the EEG is related to frequent experience of self-hypnosis, high hypnotizability, or both.