Experimental aortic intimal thickening has been induced in rabbits by sheathing the vessel with a polyethylene cuff. The alterations have been examined by light and transmission electron microscopy, during 12 months. An irregular intimal thickening develops as soon as the 15th day and includes numerous myofibroblasts with some other cells of monocytic or endothelial type. Microfibrils, elastic aggregates and collagen fibers are found in the intercellular space. Simultaneously, the media undergoes a fragmentation of the elastic laminae and the adventitia shows a capillary angiectasis and a granuloma. After 3 months there is, between the intimal smooth muscle cells, a progressive increase of elastic and collagenous material. In the media, elastic break up becomes more frequent after the 4th month and myocytes appear increasingly atrophic, which facilitates the extension of fibrosis. This is accompanied at times by a thinning of the arterial wall with or without localized disappearance of the media. All these modifications are discussed and compared to what we had previously found in the femoral artery [12].