Human platelet-derived transforming growth factor-beta (TGF-beta 1) increases the accumulation of the extracellular matrix proteins, fibronectin and type I collagen, in mesenchymal and epithelial cells. To determine the basis for this effect, we have examined the levels of mRNAs corresponding to fibronectin and alpha 2(I) procollagen in NRK-49 rat fibroblasts and L6E9 rat myoblasts treated with TGF-beta 1. TGF-beta 1 increased severalfold the levels of mRNAs for both proteins. The kinetics of this effect were similar for both mRNA species. The increase in fibronectin and alpha 2(I) procollagen mRNAs was detectable 2 h after addition of TGF-beta 1 to the cells and their maximal levels remained constant for several days. Actinomycin D, but not cycloheximide, inhibited the increase in fibronectin and alpha 2(I) procollagen mRNA levels induced by TGF-beta 1. The results indicate that TGF-beta 1 controls the composition and abundance of extracellular matrices at least in part by inducing a coordinate increase in the levels of fibronectin and type I collagen mRNAs.