SUMMARY Sixteen dogs were presensitised by multiple skin grafts prior to receiving a kidney from their specific donor. An additional four dogs received sheep kidney xenografts. A nonsensitised group of four dogs received kidney allografts only. Twelve of the presensitised, 4 of the xenografted, and 4 of the nonsensitised animals were pretreated with Arvin prior to receipt of the test kidney. Four of the presensitised dogs were not pretreated prior to kidney allografting. Determination of serum fibrinogen and whole blood-clotting times confirmed the incoagulability of the blood in Arvin-treated animals. Serial renal arterial and venous blood samples were drawn for measurement of complement levels, cytotoxic titre, and platelet and white blood cell counts. Renal arterial blood flow was continuously monitored and numerous kidney biopsies were obtained during the course of rejection. Kidney rejection times varied from 25 min to 24 hrs in allografts and from 6 to 10 min in xenografts. The pathophysiological changes of hyperacute allograft and xenograft rejection appeared to be identical, although slower to evolve and less severe in the former, consisting of decrease in renal perfusion, uptake of platelets and complement, massive platelet clumps within vessels, endothelial damage, and deposition of γ-globulin and complement in the vessels and glomeruli. Fibrin deposition was absent or diminished in Arvin-treated recipients, but the profound anticoagulation achieved by this agent neither prevented nor diminished the severity of the rejection of allografts or xenografts.