We tested the hypotheses that electrical stimulation of the fastigial nucleus increases cerebral blood flow by a dilator mechanism, impairs autoregulation during increases in arterial pressure, and attenuates increases in cerebral blood flow during acute hypertension by activation of sympathetic nerves. Cerebral blood flow was measured with microspheres in anesthetized cats during control and moderate or severe hypertension produced by stimulation of the rostral fastigial nucleus. Cervical sympathetic nerves to one cerebral hemisphere were cut to compare responses in the innervated and denervated hemispheres. Fastigial stimulation at a level that raised arterial pressure from 94 +/- 10 (mean +/- SE) to 133 +/- 6 mmHg had no significant effect on cerebral blood flow. Autoregulation was preserved because cerebral vascular resistance increased approximately 40% during the fastigial pressure response. When mean arterial pressure was raised to 189 +/- 9 mmHg by stimulation of the fastigial nucleus, cerebral blood flow increased similarly in the denervated hemisphere and the hemisphere with intact sympathetic nerves. We conclude that stimulation of the fastigial nucleus in cats does not have a direct dilator effect on cerebral vessels, does not impair autoregulation during moderate hypertension, and does not attenuate increases in cerebral blood flow during severe hypertension by activation of sympathetic pathways.