The composition and content of aortic glycosaminoglycans were studied in groups of rhesus monkeys fed control or atherogenic diets for 9 or 19 months. Aortic uronic acid content was significantly increased in both groups of monkeys with atherosclerosis. The major glycosaminoglycan in both control and atherosclerotic aortas was chondroitin sulfate with lesser amounts of heparan sulfate, dermatan sulfate, and hyaluronic acid. Dermatan sulfate was the only glycosaminoglycan to show a statistically significant elevation (65 to 87 per cent) in animals fed the atherogenic diet. This increase was positively correlated with the increased accumulation of aortic cholesterol (r = 0.4709, p less than 0.05). The results indicate that dermatan sulfate may be the major glycosaminoglycan involved during the early events of atherogenesis perhaps through retention of lipoprotein in the atherosclerotic artery.