The cerebral circulatory effects of physiological stimulation of the sympathetic nervous system have been examined in the present study. In lightly anesthetized rabbits, reflex sympathetic activation was provoked by bilateral sinus deafferentation and vagotomy. Regional cerebral blood flow (CBF) was measured by the [14C]-ethanol technique and compared in paired brain structures following unilateral superior cervical ganglionectomy. Two subgroups of hypertensive rabbits were statistically distinguished. In the first (19 of 28 rabbits), CBF in the innervated hemisphere was little modified by hypertension but there was a significant side-to-side difference in CBF between the hemispheres. In the second group (9 rabbits) CBF was markedly increased by the systemic hypertension, and little difference was noted between innervated and denervated hemispheres. We demonstrate that, during acute hypertension, the superior cervical system contributes to cerebrovascular autoregulation; this contribution varies according to the brain region studied. In a subgroup of animals, little sympathetic activity could be evidenced, and it is hypothesized that in these rabbits a vasodilatory system was activated that counteracted the myogenic, autoregulatory responses.