Extracellular matrix gene expression increases preferentially in rat lipocytes and sinusoidal endothelial cells during hepatic fibrosis in vivo. Whether parenchymal or nonparenchymal liver cells play a predominant role in the pathophysiology of hepatic fibrosis has not been firmly established in vivo. We have addressed this question by quantitating the relative abundance of specific mRNAs for collagen types I, III, and IV, and laminin in purified populations of hepatocytes, sinusoidal endothelial cells, and lipocytes from normal and fibrotic rat liver. In normal liver, type I collagen gene expression was minimal in all cell types; mRNA for types III and IV collagen were apparent in endothelial cells and lipocytes, but not in hepatocytes. Laminin mRNA was present in all cell types. Induction of fibrogenesis by either bile duct ligation or carbon tetrachloride administration was associated with a substantial increase in mRNA for types I and III collagen in nonparenchymal cells. Lipocytes from fibrotic animals exhibited a greater than 30-fold increase in type I collagen mRNA relative to normal lipocytes, and greater than 40-fold relative to hepatocytes. Type III collagen mRNA reached 5 times that in normal lipocytes and greater than 120 times that in hepatocytes. Endothelial cells exhibited an isolated increase in type I collagen mRNA, reaching five times that in normal liver. Type IV collagen and laminin gene expression were not significantly increased in nonparenchymal cells during fibrogenesis; in fact, mRNA for type IV collagen and laminin decreased by up to 50% in endothelial cells. Despite the pronounced changes that occurred in matrix gene expression in nonparenchymal cells during fibrogenesis, no change was noted in hepatocytes. We conclude that nonparenchymal liver cells, particularly lipocytes, are important effectors of hepatic fibrosis in vivo. 