Regulation of fibronectin (FN) by Platelet-derived Growth Factor-BB (PDGF-BB) and Insulin-like Growth Factor-I (IGF-I) in cultured rat thoracic aortic adventitial fibroblasts were examined. PDGF-BB enhances FN mRNA levels in a time and concentration-dependent fashion. The effect of IGF-I on PDGF-BB-induced FN levels was also examined. There was a larger increase in FN mRNA levels (4-fold) in the cells treated with both IGF-I and PDGF-BB than with either IGF-I (2-fold) or PDGF-BB (2-fold) alone. The effects of PDGF-BB and IGF-I on FN levels were examined. There was a larger increase in FN levels (135%) in the cells treated with both PDGF-BB and IGF-I than with either PDGF-BB (43%) or IGF-I (45%) alone. Regulation of adventitial fibroblast FN may lead to blood vessel diseases and/or angiogenesis.