At 24 hr following orthotopic transplantation, rat liver grafts were perfused in situ for 7 min with trypan blue, a vital dye that provides information on hepatic microcirculation and stains nuclei of nonviable cells. Spotty and uneven dye distribution was observed indicating that hepatic microcirculation was disturbed 24 hr following transplantation surgery. Under these conditions, 15-20% of the hepatocytes were nonviable as assessed from trypan blue staining and frank necrosis. In contrast, perfusion of livers from untransplanted rats or liver expiants exposed to cold ischemia for 60 min were judged normal by the criteria of uniform distribution of dye in the organ and absence of necrosis and nuclear dye uptake. Thus the observed damage was associated with reintroduction of blood and can therefore be classified as a reperfusion injury. The altered microcirculation and cell death following the operation was reduced markedly by perfusion of the cold, ischemic explant with nitrogen-saturated but not with oxygensaturated buffer for 5 min prior to the implantation operation. Protection was even greater if the perfusion medium contained verapamil (20 [mu] g\ml) a Ca++ channel blocker. We conclude that reperfusion of the stored liver causes an oxygen-dependent alteration in hepatic microcirculation that leads to hypoxia and scattered hepatocellular necrosis in the implanted graft. Brief perfusion of the hypoxic implant under anaerobic conditions may remove substrates involved in oxygen radical generation and prevent reperfusion injury upon introduction of oxygen into the graft via the blood. Taken together, these results suggest that removal of Euro-Collins' solution under anaerobic conditions may be beneficial clinically in preventing injury of surgical explants.