T w o models of orthotopic liver transplantation in the rat currently used are with and without reconstruction of the hepatic artery. The aim of this study was to assess the importance of the arterial blood supply to liver structure after orthotopic liver transplantation with advanced morphometric methods. Orthotopic liver transplantation was performed in male Lewis rats, of which 11 underwent reconstruction of the hepatic artery and 11 did not. A group of untreated controls (n = 5 ) and a group of animals with hepatic artery ligation (n = 4) were included. Eight weeks after surgery liver tissue was harvested and subjected to systematic random sampling. A point-counting method was used, volume fractions of tissue components were determined. Liver samples from rats that underwent hepatic artery reconstruction had preserved lobular architecture, and estimated bile duct (0.71% 2 0.33% [S.D.]) and connective tissue (1.89% f 0.52%) volumes were not significantly different from those of controls (bile duct, 0.34% f 0.17%; connective tissue, 0.70% f 0.07%). In contrast, liver samples from rats that did not undergo hepatic artery reconstruction showed bile duct proliferation (7.19% * 4.83%; p < 0.05) and an increase in connective tissue volume (7.54% f 3.68%; p < 0.05) associated with a decrease in hepatocyte volume (controls, 87.3% f 0.3%; rats that underwent arterialization, 85.5% f 1.0%; rats that did not undergo arterialization, 73.2% f 8.2% [p < 0.051). Interestingly, hepatic artery ligation had no significant effect on any parameter. These findings show that in contrast to orthotopic liver transplantation with arterialization, orthotopic liver transplantation without arterialization may lead to derangement of liver structure, thus demonstrating the crucial role of an intact arterial supply for the biliary tree in orthotopic liver transplantation. The lack of change with hepatic artery ligation may indicate the presence of an additional arterial supply to the bile duct under normal conditions. (HEPATOLOGY 1993; 17:3 10-3 17.)