p21(Waf1/Cip1) (hereafter referred to as p21) is up-regulated in differentiating and DNA-damaged cells, but it is also up-regulated by serum and growth factors. We show here that fibroblast growth factor-2 (FGF-2), platelet-derived growth factor (PDGF), and transforming growth factor-beta1 (TGF-beta1) all induce p21 expression in mouse fibroblasts, but with markedly different kinetics. We link their effect on p21 to Ras and mitogen-activated protein kinase kinase-1(/2) [MEK1(/2)]-regulated pathways using either a specific MEK1(/2) inhibitor (PD 098059) or cells expressing conditionally activated Ras or dominant negative Ras. We demonstrate that p21 induction by PDGF and TGF-beta1 requires MEK1(/2) and, additionally, that the TGF-beta1 effect on p21 depends on Ras, whereas the PDGF effect does not. In contrast, FGF-2 regulation of p21 is largely independent of MEK and Ras. However, PD 098059 efficiently inhibited S-phase entry of quiescent cells induced by either FGF-2 or PDGF, suggesting separate signaling pathways for FGF-2 in induction of p21 and in S-phase entry. The results suggest different but partly overlapping signaling pathways in growth factor regulation of p21.