Transforming growth factor-β (TGF-β) is involved in multiple processes including cell growth and differentiation. In particular, TGF-β has been implicated in the pathogenesis of fibrotic lung diseases. In this study, we examined regulation of the mitogen-activated protein kinase pathway by TGF-β1 in primary human lung fibroblasts. TGF-β1 treatment resulted in extracellular signal-regulated kinase (ERK) pathway activation in a delayed manner, with maximal activity at 16 h. ERK activation occurred concomitantly with the induction of activator protein-1 (AP-1) binding, a nuclear factor required for activation of multiple genes involved in fibrosis. AP-1 binding was dependent on ERK activation, since the MEK-1 (mitogen-activated protein kinase kinase) inhibitor PD98059 inhibited TGF-β1-induced binding. Induction of the receptor tyrosine kinase-linked growth factor, basic fibroblast growth factor (bFGF) protein expression temporally paralleled the activation of ERK/AP-1. Induction of AP-1 by TGF-β1-conditioned medium was observed at 2 h, similar to AP-1 induction in response to exogenous bFGF. Dependence of ERK/AP-1 activation on bFGF induction was demonstrated by inhibition of TGF-β1-induced ERK/AP-1 activation when conditioned medium from TGF-β1-treated cells was incubated with bFGF-neutralizing antibody. Together, these results demonstrate that TGF-β1 regulates the autocrine induction of bFGF, resulting in activation of the ERK mitogen-activated protein kinase pathway and induction of AP-1 binding.