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Uptake of circulating macromolecules by the aortic wall is greater downstream than upstream of branch sites in immature rabbits, but the opposite pattern is seen at later ages. The mature pattern is nitric oxide dependent; we tested whether it is also flow dependent. Intercostal arteries of anesthetized rabbits were occluded, sham operated, or left alone. Uptake of rhodamine-labeled albumin was assessed by quantitative fluorescence microscopy of the sections through the aorta. In mature animals, uptake was higher upstream than downstream of the control and sham-operated branches, but the pattern was reversed at occluded branches. In young animals, uptake was not significantly different between regions upstream and downstream of control, sham-operated, or occluded branches. The absence of the normal immature pattern may reflect an influence of anesthesia and will assist in the elucidation of mechanisms underlying this pattern. The data for mature animals provide the first direct evidence that flow determines permeability near arterial branches and may account for the inverse spatial correlation between shear stress and disease prevalence at branches of adult human arteries.