HSP47 is a collagen-binding stress protein and is assumed to act as a collagen-specific molecular chaperone during the biosynthesis and secretion of procollagen in the living cell. The synthesis of HSP47 has been reported to correlate with that of collagen in several cell lines. We examined the expression of HSP47 mRNA during the progression of carbon tetrachloride (CCl4)-induced liver fibrosis in rats. Northern blot analysis revealed that the expression of HSP47 mRNA was markedly induced during the progression of fibrosis in parallel with alpha 1(I) and alpha 1(III) collagen mRNAs. By in situ hybridization, the distribution of HSP47 transcripts was similar to that of alpha 1(I) collagen and was observed only in cells lining collagen fibrils. These collagen-positive cells were confirmed to be Ito cells by immunohistochemistry for desmin. The absence of high levels of HSP47 mRNA in the liver of rats treated with only a single administration of CCl4 indicated that the induction of HSP47 mRNA was not due to the direct effect of CCl4 as a stressor, but was due to the progression of liver fibrosis. The function of HSP47 in liver fibrosis will also be discussed.