Abstract Six rhesus monkeys prepared with electrodes implanted into numerous specific subcortical brain sites and over the brain surface under the skull were exposed to smoke of marijuana containing a significant quantity of delta 9-tetrahydrocannabinol. Electroencephalograms were obtained before, during, and after exposure to the marijuana smoke, which was delivered to the monkeys by use of a specially designed head chamber. Control agents for the study were inert marijuana of low delta 9-tetrahydrocannabinol content, tobacco, alcohol and methamphetamine. Exposure to smoke of active marijuana consistently induced distinct recording changes in the septal region, occasionally accompanied by changes in recordings from the cerebellum, postero ventral lateral thalamus, hippocampus, and orbital and temporal cortices. Only generalized electroencephalographic changes, consisting of slight shifts in the dominant frequency, were obtained in association with the other agents used in the study.