Effects of marihuana on electroencephalograms and behavior were observed in a patient in whom electrodes had been implanted into numerous predetermined deep nuclear sites and over the surface of the cortex of the brain. On four occasions while the patient was smoking a marihuana cigarette, development of euphoria was associated with the EEG appearance of distinct high-amplitude slow-wave activity (1 to 3 hertz) focal in the septal region. No significant changes were observed in EEGs recorded at other deep sites, over the cortex, or from the scalp. Nor were similar distinct changes observed in EEGs from deep sites of the brain of the same patient when effects of amphetamine, alcohol, and tobacco smoking were studied for comparison with changes induced by marihuana.