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IN previous papers [1932, 1933, 1934] we were able to show with several techniques (Stromuhr of Weese, Stromuhr of Rein, H iirthle's method) that, in dogs, the carotid sinus reflexes do not affect the cerebral vasomotor tone in an active way; indeed, lowering the carotid sinus pressure produces a passive increase of the cerebral blood supply which is due to the reflex increase of the general blood-pressure, while conversely increasing the pressure in the carotid sinus produces a passive decrease of the cerebral blood flow due to the reflex decrease of the general arterial bloodpressure.In the case of a fall of the carotid blood-pressure, the blood is diverted by means of the carotid sinus reflexes from the somatic and peripheral cephalic areas to the cerebral circulation, whose blood vessels do not participate actively in the carotid sinus reflexes. The blood flow to the central nervous system thus increases, while on the contrary the blood flow decreases, as a result of the reflex vaso-constriction, in the peripheral cephalic tissues (muscles and skin of the head) and in the somatic organs. The cerebral blood supply is thus passively dependent on the general arterial pressure, which is regulated by the carotid sinus and aorta reflexes. H. Rein [1929, 1931], on the other hand, contended that the cerebral vessels were collaborating actively in the changes of the cerebral blood flow induced by the carotid sinus reflexes. In his opinion, a decrease of pressure in the carotid sinus produces an increase of the cerebral blood flow not only in a passive way, by means of the increase of the general PH. LXXXIV.