delta 9-Tetrahydrocannabinol (THC) decreases EEG activation and causes slow waves in the cat. The EEG slow-wave activity is accompanied by a concomitant decrease in acetylcholine release from the neocortex. The findings suggest that THC depresses the brain stem activating system. Large doses of delta 8- and delta 9-THC increase brain acetylcholine levels in rodents such as the mouse and rat, but this effect is not seen with minimal doses of the cannabinoids which show behavioral effects. The most dramatic change produced by THC is that brain acetylcholine utilization is reduced primarily in the hippocampus.