Human blood platelets were labeled with 51Cr and whole blood was reconstituted. The endothelium of rabbit abdominal aorta was removed with a balloon catheter and the inverted aorta was placed on a Plexiglas rod in a perfusion chamber. The reconstituted blood was perfused through the chamber of 10 min. Radioactivity of the aorta was measured and found to be mainly caused by the labeled platelets. The platelet deposition on the damaged aorta was measured before and after ingestion of 0.5 g acetylsalicylic acid twice a day for 1 week, and was found to be reduced after the use of this drug.