The action of bilateral cervical sympathetic nerve (CNS) stimulation on mean cerebral blood flow (CBF) and on its rhythmical fluctuations was studied in normotensive rabbits by using laser-Doppler flowmetry (LDF). A reduction in mean CBF, mediated by alfa-adrenoceptors, was the predominant effect; it was more often present and larger in size in the vascular beds supplied by the carotid than in those supplied by the vertebro-basilar system. This suggests that the sympathetic action facilitates a redisrtibution of blood flow to the brain stem. The effect induced by CSN stimulation on CBF spontaneous oscillations was a consistent decrease in amplitude and an increase in frequency, irrespective of the changes produced on the mean level of CBF. The possible implications of the sympathetic actions on the state of the blood-brain barrier (BBB) are discussed. Experimental and clinical data dealing with the infuence of sympathetic activation on the cerebrovascular system have been compared. As a result the possibility of analysing the spontaneous oscillations of CBF for clinical purposes is suggested.