The aim of the study was to determine the collagen content, distribution and quantitative ratios between main types of collagen (found in normal vascular walls) in various layers of double velour DALLON grafts at different periods after implantation. Double velour DALLON prostheses were implanted into the abdominal aorta of 24 dogs. These grafts were excised 7 days, 1, 4 and 12 months after implantation and together with normal arteries, after dissection into intimal, medial, and adventitial layers, tested for their hydroxyproline content. Type I, III, V collagen was also determined (fractionation of pepsin digestion products). Already 7 days after implantation collagen appeared in all the newly-formed layers of the grafts. With the passage of time after the operation collagen content was gradually increasing. After 12 months collagen content in the intimal and adventitial layers was significantly higher than in normal arterial wall. Approximate quantitative ratio between collagen types I, III and V in all graft layers was 7:2:1 and did not change during the 1-year-observation. Rise in collagen content results in graft wall stiffening, and the presence of III type collagen increased platelet-aggregating activity. Both these factors created a tendency to the formation of thrombosis in the graft.