Collagen represents the chief structural vertebral protein. The most important function of collagen is to withstand tensile stress. The aorta is one of the important sources of collagen. The present study was undertaken to investigate the effect of high cholesterol diet on the hydroxyproline fractions and collagen content in the aorta of rabbits. 12 Weeks old NewZealand white male rabbits were divided into control group and cholesterol-fed group. The control group (n=10) was fed on 100g/day of normal diet, ORC- 4 (OrientalYeast Co. Ltd., Tokyo, Japan) for 15 weeks. The cholesterol-fed groups (the experimental groups; n=15) were fed on high cholesterol and saturated fat diet of ORC-4 containing 1 % cholesterol plus 1 % olive oil (100 g/day) for periods of 5 weeks (group 1), 10 weeks (group 2) and 15 weeks (group 3). The rabbits were sacrificed after the mentioned time period and their serum was used to determine total cholesterol, low-density lipoprotein cholesterol and triglycerides. The aortae were used to determine their hydroxyproline fractions and collagen content. Feeding rabbits on high cholesterol diet caused significant increase in serumlevels of total cholesterol, low-density lipoprotein cholesterol and triglycerides. High cholesterol diet also caused a significant decrease of collagen from the aorta which was supported by histological studies.