Histochemical and chemical studies of the acid mucopolysaccharide content of the aorta were made in rabbits fed cholesterol for up to seven months. Colloidal iron staining suggested an increase in acid mucopolysaccharide content of the aortas with mild atheromatous lesions but not when severe lesions were present. Chemical determination, however, revealed no increase in the concentration of acid mucopolysaccharide in early lesions, but did reveal a significant increase in the more severely atheromatous aortas of rabbits fed cholesterol for 6 months or more.