The permeability and ultrastructure of the intercostal orifices and interostial regions of rat thoracic aorta have been compared in the present study. The ostial regions consist of two different portions, an annulus and an inflow tract, the latter being continuous with the intercostal artery. The wall of the inflow tract has a sharp bend, demarcating inner and outer portions. Faint blueing of the lateral portions of aortic annuli is visible from 3(1/2) to 4(1/2) hours after injection of Evans blue dye and is no longer apparent at 5(1/2) hours, when there is diffuse discoloration of the thoracic aorta. Only the annuli are labeled by horseradish peroxidase (HRP) for 1(1/2) to 3(1/2) minutes after intravenous injection. There is more extensive and progressively heavier labeling of annuli and inner halves of the inflow tracts between 5 to 12 minutes. HRP is transported across aortic endothelium within micropinocytotic vesicles. Intercellular endothelial clefts do not appear to be directly permeated from the lumen through their junctions for a period of at least 3(1/2) minutes. The present observations preclude an assessment of the penetration of the junctions by HRP at later intervals. No appreciable differences are observed in the mode of transport of peroxidase through the more permeable endothelium of the annulus and less permeable endothelium of inflow tract and interostial regions. Observations made on tissues exposed to colloidal lanthanum, and on specimens stained en bloc with uranyl acetate and subjected to a combination of rotation-tilting, fail to demonstrate appreciable differences in the ultrastructure of the endothelial cell junctions of intercostal orifices and interostial regions. Since morphologic differences do not seem to account for the greater permeability of annular regions, the role of hemodynamic forces is considered. The relationship between regional variation in permeability and the development of atherosclerotic lesions is also discussed