Eight dogs were cannulated for left heart bypass. Conventional roller pumps were used for bypass with one of the pump console modified to deliver pulsatile bypass. The experiments were performed at normothermia. Each dog was used as its own control. For measurement of cerebral blood flow (CBF) Xe133 was injected into vertebral artery and the gamma activity elimination over the skull was registered. In addition blood samples were withdrawn from the axillary artery and the sagittal sinus for measurement of blood gases. The CBF to the gray matter of the brain was significantly higher during the periods with pulsatile perfusion (P less than 0.01). This was probably due to decreased total cerebral vascular resistance (P less than 0.001). Mean aortic pressure also decreased when the mode of perfusion was changed from non pulsatile to pulsatile (P less than 0.05), indicating a general decrease of the peripheral vascular resistance. A good correlation was noted between the arteriovenous oxygen saturation difference (AV-O2 diff) across the brain and the CBF (R = -0.77, P less than 0.01).