In recent years, unilateral or bilateral multiple stenoses in the internal carotid artery have been increasing in number. Full understanding and complete analysis of these hemodynamics are, however, sometimes difficult because multiple factors such as number, size and location of the stenotic lesions, and capacity of collateral circulation differ with individual cases and are related mutually in complex fashions. One of the effective means to solve these problems is to study the hemodynamics and to simulate their changes after an expected operative procedure by the use of a hydraulic vascular model, where any factor can be managed independently. A vascular model of the internal carotid artery (ICA) has been manufactured with silicone and glass tubes in similar dimensions to averaged adults. Peripheral vascular resistance (Rp) is so adjusted as to obtain an arterial flow of 180 ml/min at an intraluminal pressure of 60 mmHg. Four kinds of stenosis segments, Ra (2.59 mm in diameter), Rb (1.94 mm), Rc (1.12 mm) and Rd (0.84 mm) are prepared and used in various models simulating unilateral multiple stenoses, unilateral solitary long stenosis and bilateral multiple stenosis. The results obtained are; 1. In case of two stenoses of different degree arranged in series, advanced one was found to have a dominant effect on flow reduction. This implies that only its removal can results in good augmentation of flow in unilateral multiple stenoses of the ICA. 2. In case of more than three stenoses of the some degree arranged in series, increase in flow can not be obtained until the last one is cleared.(ABSTRACT TRUNCATED AT 250 WORDS)