In a study on the pathogenesis of arterial wall thickening, hemodynamic factors in the common carotid artery of the rat were experimentally altered with an autograft. The relationship between flow pattern and wall-thickening was examined in a half-ring bypass model with an induced stenosis, using both flow-visualization in a corresponding in vitro model circuit and observation of the wall by microscopy. Wall-thickening was found in the neighborhood of bifurcations, junctions and curved segments, which corresponded to regions of low-shear in the flow-field. Marked histologic changes in the wall were observed in the post-stenotic segments where the flow field was very disturbed. Histologic changes in the arterial wall correlated well with flow patterns.