These experiments examined behavioral inhibition in rabbits by comparing low and high frequency electrical stimulation of the septum and the caudate nucleus on 3 measures of unlearned activity, a discriminative active avoidance task, and a lever-press operant task. Four and 8/sec stimulation of either site evoked a sustained rhythmic discharge in the hippocampus morphologically similar to naturally occurring theta rhythm. At 100/sec, the EEG was persistently desynchronized by septal stimulation and generally unaffected by caudate stimulation. Septal stimulation at all frequencies caused massive increases in exploratory activity in some rabbits but had no clear effect on jiggle stand activity or on duration of immobility reflex (animal hypnosis). Both septal and caudate stimulation inhibited previously learned avoidance and operant behavior, and these effects were frequency specific occurring at low, but not high, stimulus frequencies. Caudate stimulation also inhibited jiggle stand activity and exploration, and enhanced the immobility reflex. © 1975.