Discordant xenografts are hyperacutely rejected within minutes. Disturbances in the microcirculation are considered to be the central mechanisms of hyperacute xenogeneic rejection (HXR). In this study intravital fluorescence microscopy was applied to investigate the dynamics of microcirculatory alterations in a setting in which HXR was inhibited by complement (C) depletion. Blood flow was measured as rat livers were perfused with isogeneic rat or xenogeneic human blood to assess the pattern of either physiological isogeneic hemoperfusion or in the course of HXR. Next, the complement system of the perfusate was inactivated by cobra venom factor (CVF) in order to inhibit HXR. Liver sinusoids of the isogeneic group were homogeneously perfused (sinusoidal perfusion rate 93.6+/-0.3%), whereas in the xenogeneic group the sinusoidal perfusion rate dropped to 67.1+/-3%. The perfusion in the periportal zone of an acinus was significantly lower ( 59.0+/-3.3%) than in the pericentral zone (76.2+/-3.1%). Treatment with CVF improved the sinusoidal perfusion to a value of 85.6+/-2.3%, physiological perfusion, however could not be reached. In contrast to the isogeneic group, massive white blood cell (WBC) and platelet accumulation was found in the xenogeneic group, especially in the terminal portal vessels and in the periportal zone of liver acini. WBC and platelet counts show that the adherence of these cells appears rapidly in the first 5 min after reperfusion as firm adherence. CVF was not able to inhibit WBC and platelet accumulation, indicating that WBC endothelial interactions do not require an intact complement system. Bile flow, a parameter of liver function, decreased only slightly during isogeneic perfusion. The addition of CVF to the rat blood reduced the bile flow to one half of the untreated isogeneic flow, indicating a hepatotoxic side-effect of CVF. In xenogeneic perfusion the bile flow dropped to 62.6% and with the addition of CVF to 37.5% in the first 15 min after reperfusion. The bile flow of the CVF treated groups recovered during the perfusion but could not reach isogeneic values.