The influence of aortic coarctation on the pattern of Evans Blue dye uptake by the aorta has been examined. Modification of aortic haemodynamics associated with the experimental production of aortic coarctation resulted in three changes in the dye-uptake pattern; first, an increased area of dye uptake in the aortic arch proximal to the coarctation; second, decreased bluing distal to the coarctation; and third, the appearance of new small areas of dye uptake a short distance below the coarctation. The latter were considered as early jet lesions, probably the result of a direct jet injury to the endothelium. The area of dye uptake proximal to the coarctation was greatly increased when the coarctation was tight (90% reduction in aortic cross-sectional area), but was not modified appreciably when the degree of narrowing was less. Reduction of the spontaneous focal bluing pattern distal to the coarctation was observed in the presence of both tight and loose coarctations. The relationship of these modifications of aortic Evans Blue uptake to the altered haemodynamics associated with aortic coarctation, endothelial injury, and permeability are discussed. It is concluded that the haemodynamic changes associated with aortic coarctation are capable of modifying the aortic uptake patterns of the protein-binding azo dye Evans Blue. It is suggested that the spontaneous focal patterns of aortic Evans Blue uptake might also be determined by focal haemodynamic factors.