It was attempted to irritate electrically the carotid sinus nerve in dogs under morphine or ether and in cats under urethane or ether recording the mean arterial pressure, and in fact when the heart was wholly normally innervated, then after double vagotomy, further after the stellate and inferior cervical ganglia had been bilaterally removed, and finally after the thoracic sympathetic chain to the VIII. ganglion had been excised on both sides. The strength of the tetanizing current was chosen as somewhat stronger than that first felt on the tongue, and the same strength was usually held during the whole experimental course of one individual. When the heart innervation was wholly intact, stimulation of the carotid sinus nerve occasioned in dogs and cats always a blood pressure fall and a slow rate of the heart. Both effects were distinctly larger in dogs than in cats, and the use of morphine in dogs exaggerated them. After double vagotomy the sinus nerve stimulation produced invariably in dogs again a blood pressure fall while in cats the reaction visible in the pressure varied; either the pressure fell only, or rose only or further elevated at once, but was soon followed by a drop. The first kind of reaction appeared rather infrequently and the latter most frequently. The pulse rate frequently remained unaltered, and rather seldom retarded in dogs, and in addition an acceleration was seen not seldom in cats, different features compared with the findings in rabbits, yielded in the hands of previous experimentalists of the depressor nerve. The variation in rate was on a far smaller scale compared with the animals with the heart intactly innervated. There were detectable some evidences for assuming the part played by the reflex vagus action in lowering the blood pressure in dogs especially under morphine and in cats. The fluctuations in the blood pressure in cats after double vagotomy, above mentioned, by the sinus nerve stimulation were readily changeable to some extent, for example by regulating the strength of the stimuli applied. Further bilateral removal of the stellate and inferior cervical ganglia and moreover of the thoracic sympathetic chain to the VIII. ganglion modified no more markedly the response of the blood pressure and of the heart rate to the sinus nerve stimulation. In dogs the blood pressure was lowered on a similar scale as before and the heart rate reduced only seldom and slightly, but usually was unaltered; only when the heart was completely denervated was an acceleration in the pulse frequency detected in a few cases. And in cats various forms of the blood pressure fluctuation occurred, and several kinds of variations in the heart rate appeared as before, only an acceleration seemingly decreased in a number of cases after ganglionectomy. Occasionally there was detectable a trace of the role played by the cardiac sympathetic in causing the change in the heart rate by the sinus nerve stimulation. To recapitulate: Thus the slowing in the heart rate in dogs and cats, reflexly elicited by stimulating the carotid sinus nerve, is chiefly or sometimes solely intermediated by the vagi; there is only occasionally a trace of taking share of the cardiac sympathetic in the carotid sinus reflex re the heart rate. There are also evidences for assuming the reflex vagus action as responsible for a portion of the blood pressure fall caused by way of the carotid sinus reflex. After double vagotomy the blood pressure in dogs and cats behaves quite differently in response to the sinus nerve stimulation.