It has been postulated that GABA acts as an inhibitor of command neurons, the activity of which initiates behavior. A prediction of this hypothesis is that elevations in functional GABA levels in the brain will cause decreases in behavioral output. Accordingly, in this study rats were injected intracisternally with either saline or one micromole of GABA or its excitatory precursor, glutamic acid, and behavioral activity in a novel environment was recorded as it habituated over the course of the subsequent 4 hours. The activity of the animals that were injected with GABA was greatly decreased, while the activity of the animals that were injected with glutamic acid was apparently unaffected, as compared to animals given saline. These data provide support for the hypothesis that GABA functions as an inhibitory neurotransmitter for behavior-activating command neurons.