Increasing evidence implicates injury of hepatic sinusoidal endothelial cells as an important component in the development of several forms of liver injury. The purpose of this study was to test the hypothesis that alcohol [ethanol (EtOH)]-induced pathological changes of the sinusoidal endothelial cell of the liver precede, and may lead to, hepatocyte injury. BALB/c mice were treated with EtOH either acutely (1.5 or 3.0 g.kg-1 body weight, i.p.) or chronically [by feeding an EtOH-containing (4%, w/v) liquid diet]. Acutely treated animals were killed 3, 6, and 12 hr after EtOH administration, whereas chronically treated animals were killed 12, 28, and 56 days after the initiation of EtOH feeding. The levels of plasma EtOH, hyaluronan (a functional marker for sinusoidal endothelial cell), and alanine-2: oxoglutarate aminotransferase (ALT) activity (a marker for hepatocyte damage) were measured in all groups. The livers were examined by electron and light microscopy. Between 3 and 6 hr after intraperitoneal injection of EtOH, the plasma EtOH levels were relatively stationary (5 and 11 mM for the low- and high-dose groups, respectively). At 12 hr, EtOH was almost completely cleared from the plasma. Hyaluronan levels were increased 3 hr after EtOH exposure at both doses and reached a peak at 6 hr after EtOH administration. In the low EtOH dose animals, the hyaluronan level declined toward normal values at 12 hr. In the high EtOH dose group, hyaluronan levels were still above normal values 12 hr after EtOH administration. No changes in the plasma ALT level were observed in either acutely EtOH-treated groups. In animals treated chronically, plasma hyaluronan levels were markedly increased at 12, 28, and 56 days of EtOH feeding. Plasma ALT levels were elevated at 28 and 56 days, but not at 12 days, of EtOH feeding. Scanning electron microscopy of the liver sinusoid in the acutely treated animals showed the presence of large gaps co-existing with normal sieve-plate fenestrations in sinusoidal endothelial cells. Such changes were seen 3 hr after the high dose and 6 hr after the low dose of EtOH. They disappeared 12 hr after low dose, but lasted well beyond this time point after the high dose of EtOH. Twelve days after the start of EtOH feeding, no changes in the electron microscopic appearance of the sinusoid could be observed. However, 26 days after the initiation of EtOH feeding, the sinusoidal endothelial cells displayed a reduced number of fenestrae. Moreover, the remaining fenestrae were distributed uniformily rather than in organized sieve plates. In addition, at these latter time points, transmission electron microscopy demonstrated the presence of fibrous material in the space of Disse. Both light and transmission electron microscopy demonstrated the presence of lipids within the hepatocyte. The picture observed 56 days after the start of EtOH feeding was essentially the same as at 28 days, except that the reduction in the number of fenestrae was more accentuated. These data document EtOH-induced pathological changes in sinusoidal endothelial cell before either biochemical or histological hepatocyte damage.