Collagen and elastine of the aorta were estimated in normal chickens of both sexes in cockerels, gonadectomized or treated with estradiol, and in hens treated with testosterone.The results showed that collagen and elastine were significantly higher in males than in females. Gonadectomy in males decreased significantly the content of collagen and elastine, so that the values became similar to those observed in females. The treatment of males with estradiol lowered the collagen and elastine content to values similar to those observed in females and in gonadectomized males. The treatment of females with testosterone increased significantly the collagen and elastine content of the aorta to levels similar to those observed in males.