The spatial distribution of sites of enhanced permeability to the macromolecule horseradish peroxidase (HRP) in the normal rabbit aorta after one min circulation was studied using image analysis. These sites, referred to as "HRP spots," exhibit a nonuniform distribution that is qualitatively similar in all rabbits studied. The density of HRP spots is highest in the aortic arch, decreases distally, reaches a minimum in the lower descending thoracic aorta, and then increases again in the abdominal aorta. The region of highest spot density follows a clockwise helical pattern in the aortic arch and outside the arch occurs in streaks largely oriented in the bulk flow direction. The streaks in the abdominal aorta localize along the anatomical right lateral wall and occasionally along the left lateral wall proximal to the celiac artery and along the ventral wall between the celiac and superior mesenteric arteries. The density of spots is high in the immediate vicinity of aortic ostia with the most elevated density being distal to ostia in most cases. At a short distance from the ostium edge of the celiac and superior mesenteric branches the proximal density is comparably high, and no preferred spot orientation is observed around the brachiocephalic vessel. These results are consistent with an influence of localizing factors such as detailed hemodynamic phenomena and/or arterial wall structural and/or functional variations.