We measured cerebral blood flow using both the radioactive microsphere technique and the cerebral venous outflow technique in dogs anesthetized with chloralase. The effect of sympathetic stimulation on cerebral blood flow was observed during both normocapnia and prolonged hypercapnia using both blood flow techniques. The increase in blood flow with hypercapnia was the same with both methods. During hypercapnia the venous outflow method showed a 38% decrease and microspheres an 18% decrease in cerebral blood flow with sympathetic stimulation. At normal CO2, stimulation caused a decrease in cerebral venous flow: no change was observed with the microsphere method. Analysis of the blood flow patterns to extracerebral tissues and evaluation of extracerebral arterial reference samples failed to prove the existence of axial streaming and subsequent skimming of microspheres within the cephalic circulation. It is concluded that direct electrical stimulation of the sympathetic innervation of the cerebral vessels is capable of reducing cerebral blood flow even during a profound hypercapnic vasodilation.