The molecular mechanism involved in the expression of collagens by hepatocytes were investigated in both pure and co-culture with another rat liver epithelial cell type (RLEC). We measured the steady-state levels of mRNAs coding for pro alpha 1(I) and pro alpha 1(IV) chains by Northern analysis and by dot blotting, using specific recombinant cDNA probes. In freshly isolated hepatocytes, only small amounts of pro alpha 1(I) and pro alpha 1(IV) mRNAs were detected by dot-blot analysis. After 3 days in culture, the pro alpha 1(I) and pro alpha 1(IV) mRNA levels increased 2 to 5 times. The amount of pro alpha 1(IV) mRNAs was identical in hepatocyte cultured with RLECs while the pro alpha 1(I) mRNA level was 5 times that in pure hepatocyte culture. Hydrocortisone reduced pro alpha 1(I) mRNA in hepatocyte cultures, but had no effect on co-cultured cells. In both culture systems, this glucocorticoid did not act on the steady-state pro alpha 1(IV) mRNA level. Whatever the age and the type of culture (pure or mixed) RLECs exhibited the highest levels of pro alpha 1(I) and pro alpha 1(IV) mRNAs, which were reduced by hydrocortisone. These results show that procollagen gene expression by hepatocytes is not directly correlated with their functional state and that corticosteroids differently affect the expression of different collagen genes and collagen deposition.