The effect of antiaggregating agents - acetylsalicyclic acid (ASA) and dipyridamol - on the rejection of allotransplanted canine kidneys in presensitized animals were studied. Blood flow dropped more markedly in the control group than in the group treated with ASA and dipyridamol. 24 hours after transplantation the blood flow in this group was significantly higher than in the control group (p 2,5 %) and in the third group treated with dipyridamol alone. The cellular rejection started earlier and was more pronounced in the control group. Only in this group vascular rejection and microthrombi were demonstrated. Urine output ceased 24 to 60 hours after transplantation. The immunosuppressive, antiphlogistic and platelet antiaggregating effect of the combined ASA and dipyridamol therapy is thought to be responsible for the better results in this group. In this experimental model the combined treatment of ASA and dipyridamol results in a later onset of cellular rejection, in better blood-flow of the kidney and later onset of anuria.