We have shown that the G protein-coupled receptor (GPCR) agonists, thrombin and Factor Xa, stimulate smooth muscle cell (SMC) proliferation through transactivation of the EGF receptor (EGFR) or the FGF receptor (FGFR), both of which are tyrosine kinase receptors. In the present study, we investigated whether platelet-derived growth factor (PDGF), a tyrosine kinase receptor agonist, might transactivate another tyrosine kinase receptor to induce SMC proliferation. Because heparin inhibits PDGF-mediated proliferation in human SMCs, we investigated whether the heparin-binding growth factor basic fibroblast growth factor (bFGF) and one of its receptors, FGFR-1, play a role in the response of human arterial SMCs to PDGF-BB. PDGF-BB induced the release of bFGF and sustained phosphorylation of FGFR-1 (30 minutes to 6 hours). A bFGF-neutralizing antibody inhibited PDGF-BB-mediated phosphorylation of FGFR-1, DNA synthesis, and cell proliferation. In the presence of bFGF antibody, PDGF-BB-induced early activation of ERK (0 to 60 minutes) was not affected, whereas late ERK activation (2 to 4 hours) was reduced. When FGFR-1 expression was suppressed using small interfering RNA (siRNA), ERK activation was reduced at late, but not early, time points after PDGF-BB stimulation. Addition of bFGF antibody to cells treated with siRNA to FGFR-1 had no further effect on ERK activation. Our results provide support for a novel mechanism by which PDGF-BB induces the release of bFGF and activation of FGFR-1 followed by the sustained activation of ERK and proliferation of human SMCs.