Periodic breathing was induced in dogs under morphine and urethane anesthesia by moderate occlusion of the cerebral blood vessels. This was accomplished by placing one of the common carotid arteries in a compression chamber, thus making possible partial or complete occlusion of this artery. The remaining cerebral arteries were occluded and de-occluded as conditions required. Pulmonary ventilation and mean blood pressure were recorded. Volume flow of blood was followed in the carotid and femoral arteries thermoelectrically with small thermopile vessels of short lag. The vertebral and external jugular venous flows were recorded by the enclosed drop method and intestinal venous flow by the external drop method. Periodic changes occurred in mean blood pressure, in femoral and carotid arterial blood flows, in vertebral, external jugular, and splanchnic venous flows, and in depth and rate of respiration. Changes in blood flow were synchronous with the periodic variations in mean blood pressure. In contrast to this general periodicity there were no obvious rhythmical changes in heart rate. Ventilation increased during decreasing blood pressures and blood flows, and decreased with increasing pressures and flows. With the conditions provided no apneas developed. The absence of periodic changes in heart rate suggested that the changes in mean blood pressure may be due to periodic activity of the vasomotor center. It is concluded that the periodic changes in ventilation are related to the periodic changes in the circulatory system but since volume flow of blood and mean blood pressure vary with each other the periodic changes in ventilation may be a result of periodic chemical changes in the respiratory center produced by fluctuations in blood flow or of the reflex effects of periodic stretching and contraction of the aorta and of the carotid sinus. The relative importance of these 2 effects cannot yet be evaluated.