Abstract Some of the electrophysiological and behavioral characteristics of isolated midbrain preparations are described. The isolation was achieved posteriorly by pretrigeminal transection, and anteriorly by premesencephalic transection or a cut paralleling the optic tract and ending in the preoptic area. These preparations displayed comparable activity. Ocular fixation reflexes were absent even in those preparations having optic input, and stable conditioned reflexes could not be formed. Most striking, however, is the almost total absence of background activity in the mesencephalic reticular formation. This depression does not arise from circulatory insufficiency, “shock” or cell death since: (a) the pupils remain miotic, (b) electrical stimulation of the mesencephalic reticular formation yields pupillary dilatation and eye movements, (c) no severe degeneration is seen in chronic preparations, (d) seizure activity is often displayed in both acute and chronic conditions, and (e) background activity is not much depressed in the pretectal area and superior colliculus even with the additional elimination of optic input. It is concluded that background activity of the mesencephalic reticular formation is dominated almost exclusively by input from the ipsilateral forebrain.