Age related changes in thoracic aorta (TA) and abdominal aorta (AA) of male APA hamsters from 3 to 12 months of age were examined morphometrically and ultrastructurally. The nuclear density of smooth muscle cells (SMCs) was larger in AA than in TA, and it decreased with advancing age. In contrast, the collagen fibre density was larger in TA than in AA, and it increased correlatively with aging, especially in TA. Electron microscopic examinations revealed that subendothelial cystic spaces and aggregations of fragments of elastic and collagen fibres were found at 3 months of age and progressed with advancing age in TA, while they were not evident in AA even at 12 months of age. Irregularity of medial SMC contours and an amount of SMC-associated collagen fibres were more prominent in TA than in AA throughout the experimental period. Degenerative changes of endothelial cells and medial SMCs progressed with aging in both TA and AA, and degenerated SMCs were characterized by aggregations of swollen mitochondria.