The brainstem of anesthetized cats was electrically stimulated to examine the changes in the intracranial pressure (ICP). There were pressor and depressor sites, which preferentially produced an immediate increase and decrease in ICP in association with the arterial pressor and depressor responses, respectively. A preferential increase in ICP was also observed by stimulation of some depressor sites. The stimulus-induced ICP responses were usually different from the secondary ICP changes due to nonneurogenic alteration of arterial blood pressure (BP) as evoked by arterial bleeding and infusion of saline solution; the stimulus-induced increase in ICP was greatly enhanced when the stimulation to the pressor sites was applied at lowered BP levels and at moderately elevated ICP levels. In addition, when a gradual elevation in ICP was spontaneously observed with the lowering of the BP level, the pressor site-induced increase in ICP exceeded 70-100mmHg at the peak plateau-like waves, regardless of the magnitude of accompanying arterial pressor response. We propose that the stimulus-induced ICP responses cannot be explained merely by the metabolic changes, the decreased intracranial compliance, and the secondary transmural action on the intracranial space of the arterial pressor and depressor responses. A neurogenic mechanism that directly affects intracranial blood diameter may be involved in the ICP responses, especially those observed, at a lower BP level, in addition to extracranic action of sympathetic and parasympathetic nerve activities.