Types I, III, and IV collagens were localized immunocytochemically in muscular and small arteries of fetal bovine tissues and in different regional segments of the aorta. The distribution of these collagens was found to be unique, not only with respect to collagen type and fiber orientation but also to the specific region of the vessel being examined. Collagen types I, III, and IV often codistribute in the intimal and medial regions of blood vessels, whereas types I and III show selective association in fibers with varied morphology in the adventitial layer. Specific segments of the aorta (proximal thoracic versus distal abdominal) were examined to determine the relative distribution and morphologic presentation of the collagens. In the ascending region of the aorta proximal to the heart, the types I, III, and IV collagen colocalized in the intimal and medial layers. Types I and III collagen predominated in the relatively small adventitial layer, whereas type IV collagen was found only in association with the media of small blood vessels and capillaries in this layer. In the descending thoracic region of the aorta distal to the arch, types I and IV collagens were distributed throughout the intimal and medial layers, whereas type III collagen localization was variable depending on the antibody used for detection. The adventitia stained predominantly for type III collagen that was distinctly organized in folded arrays of fibers. These same fibers stained less intensely with antibody to type I collagen. The abdominal aorta near the common iliacs stained in a fashion similar to that of muscular arteries of the organs and tissues examined, i.e., types I and IV collagen were found in the intimal and medial layers, whereas type III collagen localized heavily to the adventitial layer, with differential staining in the intima and media depending on the antibody used. Additional staining of the adventitial region of the abdominal aorta with type I and type IV collagen antibodies was the same as in the proximal and descending regions. Since the abdominal aorta sustains decreased levels of pulsatile distension compared with the ascending thoracic aorta, type III collagen distribution and its association with other matrix components may be important in regulating the range of distensibility of the vessel wall.