Recent studies have suggested an association between tumor necrosis factor-α (TNF-α) and the development and progression of acute liver failure. To investigate the role of TNF-α in the mechanism of massive hepatic necrosis, we studied a mouse model of TNF-α and D-galactosamine (GalN) -induced hepatic necrosis by ultrastructural analysis. Administration of GalN caused edema of hepatocellular microvilli and widening of sinusoidal endothelial fenestrae (SEF); administration of TNF-α caused only a widening of the SEF. Massive hepatic necrosis with hemorrhage was seen 6 hr after concomitant administration of TNF-α and GalN. In the ultrastructural analysis, edema of the hepatocellular microvilli, widening of the SEF, and transmigration of red blood cells (RBC) and platelets to the space of Disse without exfoliation and necrosis of the sinusoidal endothelial cells were observed. Fibrin deposits were seen in areas adjacent to injured hepatocytes. The diameter of the SEF was significantly greater than in the nontreated group and the groups treated with TNF-α or GalN alone. These results suggest that as a consequence of the increase in diameter of the SEF, transmigration of RBCs and platelets to the space of Disse may have resulted in massive hepatic necrosis due to occlusion of the microcirculation.