In experiments in dogs we studied the effect of antiaggregating agents on the patency of prosthetic vascular grafts (4 mm in diameter) and vein grafts, both placed in areas with a rate of flow ranging between 50-60 ml/min. All 6 prosthetic grafts became occluded in the nonmedicated controls, 1 of 5 prosthetic grafts remained patent for a prolonged period of time in dogs receiving 250 mg acetylsalicylic acid 3 times a day, while 5 of 6 prosthetic grafts remained patent in animals receiving an antiaggregating mixture. Follow-up studies showed that antiaggregating medication had no effect on the patency of vein grafts, but it prevented the formation of mural thrombi and subendothelial proliferation.