Recently Hatahara and Seyer (Hatahara, T., and Seyer, J.M. Biochim. Biophys. Acta (1982) 716, 377-382) isolated a factor from fibrotic rat liver which stimulates collagen synthesis in cultured fibroblasts without affecting their rate of proliferation. To investigate the mechanism of fibrogenic factor-mediated enhancement of type I collagen synthesis, we quantitated the levels of mRNAs coding for pro-alpha 1(I) and pro-alpha 2(I) chains in rat dermal fibroblasts. Cell-free translation experiments revealed that the fibrogenic factor caused greater than 5-fold increase in the translatable levels of type I mRNAs. We also quantitated collagen mRNAs by techniques of Northern blotting of glyoxylated poly(A+) RNA followed by hybridization to nick-translated human cDNA clones containing the coding sequence of pro-alpha 1(I) and pro-alpha 2(I) chains. Furthermore, we investigated the relative rates of collagen mRNA transcription in the isolated nuclei of treated and control fibroblasts. Similar quantitation of beta-actin mRNA transcription, which remains unaffected by the treatment with fibrogenic factor, was used as an internal control. We demonstrate that the fibrogenic factor causes a 4-6-fold increase in the rate of transcription of pro-alpha 1(I) and pro-alpha 2(I) genes. Finally, we also show that the rate of intracellular degradation of collagen is not significantly altered in cells treated with fibrogenic factor. These results combined with data on cell-free translation strongly suggest that the increased accumulation of type I collagen mRNA in fibrogenic factor-treated fibroblasts is a consequence of enhanced rates of collagen mRNA transcription.