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The animal model of hepatic fibrosis induced by bile duct ligation represents an experimental model of human chronic biliary fibrosis. Much attention has been given to the hepatic stellate cell (HSC), or perisinusoidal cell, as the source of the extracellular matrix proteins. However, in the bile duct ligation model, mesenchymal cells other than HSC may be involved in the early stages of fibrosis development. The current study examined, in Sprague-Dawley rats, proliferation in different liver cell subpopulations as well as expression of alpha-smooth muscle (SM) actin and desmin in portal fibroblasts and HSC at 6 hours and 1, 2, 3, and 7 days after bile duct ligation. Kinetics of liver cell proliferation and of phenotypic modulation of portal fibroblasts and HSC (expression of alpha-SM actin and desmin) was evaluated by immunocytochemistry, immunofluorescence, and immunoelectron microscopy using immunogold technique. In sham-operated animals, the evaluation of proliferation in various liver cell subpopulations revealed nonsignificant changes compared with nonoperated rats. alpha-SM actin was detected in vessel walls but was absent in cells of portal tract and parenchyma. Desmin was expressed in vessel walls and in some fibroblastic cells of portal stroma (8.2 cells/unit area) as well as in HSC in acinar Zones 1 and 3 (15.6 cells/unit area and 7.1 cells/unit area, respectively). In bile duct-ligated rats, 24 and 48 hours after ligation, marked proliferations of bile duct epithelial cells (labeling indices 36.8% and 29.5%, respectively) and of periductular fibroblasts (labeling indices 16.7% and 31.0%, respectively) were observed; thereafter, proliferation decreased for both populations (labeling indices at 7 days 12.0% and 11.6%, respectively). HSC proliferation increased gradually until the third day (labeling index 18.6%) and then leveled off. Immunocytochemistry and immunoelectron microscopy revealed a significant number of cells expressing alpha-SM actin 72 hours after bile duct ligation in the stroma adjacent to proliferating ductules. The number of alpha-SM actin-positive cells increased until the seventh day (251.6 cells/unit area). At all times examined, the distribution of alpha-SM actin was restricted to the connective tissue stroma adjacent to proliferating ductules; alpha-SM actin was not expressed in HSC of the lobule. An expansion of desmin expression was noted in fibroblastic cells in stroma surrounding proliferating ductules until 72 hours after bile duct ligation (74.7 cells/unit area) followed by a plateau. At this time, desmin expression increased also in HSC; as in controls, the number of positive cells was greater in Zone 1 (31.8 cells/unit area) than in Zone 3 (18.5 cells/unit area). Double immunofluorescence staining detected by confocal microscopy showed that the majority of portal fibroblastic cells expressing alpha-SM actin was desmin negative 48 hours after bile duct ligation. From 72 hours, portal fibroblastic cells coexpressing alpha-SM actin and desmin appeared, and their proportion increased until 7 days. The present findings indicate that in the early phase of bile duct ligation, there is a marked and transient proliferation of bile duct epithelial cells associated with proliferation of portal periductular fibroblasts, which rapidly express alpha-SM actin. This fibroblastic population may play a dominant role in the early portal fibrosis after bile duct ligation.