Decreased pulsatile blood flow may occur in certain clinical conditions in which congenital or acquired constriction is present in a major artery. Coarctation of the aorta, renal arterial stenosis, and arteriosclerotic narrowing of visceral and peripheral arteries may produce significant dampening of the pulse wave distal to the site of partial obstruction. Diminished pulsatile blood flow is also present during open heart surgery when roller pumps are used in the extracorporeal circuit. The optimum form and amplitude of the pulse wave is important in the development of prosthetic indwelling mechanical hearts. These considerations prompted us to study the vascular response to pulsatile and nonpulsatile blood flow. The resistance of the systemic and pulmonary arterial beds to each type of perfusion was investigated in dogs. Methods Group A.— A left thoracotomy and total cardiopulmonary bypass were performed upon 40 dogs anesthetized with 5% thiopental sodium (Fig 1). Adequate ventilation with oxygen