Turnover and accumulation of collagen and elastin in aorta were determined in ovariectomized female rats receiving either estradiol replacement therapy or oil vehicle injection (control rats) for three weeks prior to study. In the turnover experiments rats from both groups were administered 14C-proline ip and the SA of hydroxyproline in both collagen and elastin of aorta was determined at 3, 24, 48 and 144 hr after administration of the isotope. In the accumulation experiments rats were killed at the end of the three week treatment period and total collagen, percent collagen, total elastin, percent elastin, and collagen to elastin ratio in the aortas were determined. In the rats treated with estradiol specific activity of hydroxyproline in both collagen and elastin declined more rapidly than in oil treated rats. Control rats were heavier, had heavier aortas, and had significantly greater total collagen, percent collagen, and collagen to elastin ratio in aortas than did estradiol treated rats. It is concluded that estradiol increases degradation of both collagen and elastin in rat aorta, resulting in a lower total accumulation of both these fibers but a higher proportion of the elastic fibers, thus decreasing the stiffness of the vessel wall. (Endocrinology91: 1227, 1972)