AbstractWe characterized the structural changes of sinusoidal endothelial cells in chronic ethanolfed rats and rats with cirrhosis induced by thioacetamide. The phenotypic changes of sinusoidal endothelial cells in fibrotic rats induced by thioacetamide and the reversibility of these changes were also investigated under transmission and scanning electron microscopy, regular microscopy and by immunohistochemistry with laminin and von Willebrand factor antibodies. The diameter and porosity of sinusoidal endothelial fenestrations were increased in chronic ethanol‐fed rats without liver fibrosis, however, they decreased within 4 weeks of the cessation of thioacetamide treatment. A basement membrane‐like structure in Disse's space was noted 6 weeks after thioacetamide treatment. Laminin was detected in Disse's space after 4 weeks and von Willebrand factor was detected in the cytoplasm as granular fluorescence after 6 weeks of thioacetamide treatment. Reversibility of the phenotypic changes of the sinusoidal endothelial cells was demonstrated in fibrotic liver of rats that received thioacetamide for 6 weeks after long‐term discontinuation of thioacetamide administration. These results indicate that the structural and immunohistochemical characteristics of sinusoidal endothelial cells change in chronic ethanol‐fed rats and fibrotic rats and these changes are rev