A light and electronmicroscopic study in the intima of the smaller and larger curvatures of a healthy rabbit aortic arch was performed. In both curvatures there were areas of intimal thickenings of varying width, and consisting mainly of collagenous and elastic fibers and smooth muscle cells. The structure of the wider thickenings resembled the media layer. There was evidence that such intimal thickenings are media growth in thickness and physiological in nature. It is inferred that the early lipid deposition in the intimal thickenings of the smaller, but not in the larger curvature, is not related to morphological differences, since both curvatures had the same intimal pattern.186167-72