Elevated expression of transforming growth factor (TGF)-β 1 has been implicated in prostate tumorigenesis despite its growth-inhibitory effect on normal epithelial and carcinoma cells of the prostate. In this study, we identified that G₁-to-S transition of the cell cycle is stimulated by TGF-β1 in the prostate cancer cell line TSU-Pr1. No mutation of signal mediators, including Smads, and induction of PAI-1transcription indicated that the TGF-β1 signaling cascade is functionally intact in this cell line. Whereas pharmacological inhibitors of various mitogenic signaling pathways showed no effects,blockade of the mitogen-activated protein kinase (MAPK) pathway by the MAPK kinase 1 inhibitor PD98059 restored the growth inhibitory role of TGF-β1 in TSU-Pr1, which carries an oncogenic mutation in Ha-Ras (V12). Moreover, expression of antisense Ha-Ras or dominant negative Raf-1abrogated the mitogenic effect of TGF-β1 in TSU-Pr1, and the TGF-β1 inhibition of DU145 was switched to stimulation by V12Ha-Ras transfection. Whereas the negative growth regulation by TGF-β1 was completely inhibited by dominant negative Smad2, Smad3, or Smad4,its mitogenic effect was not affected, suggesting that this action is Smad-independent. Interestingly, whereas the TGF-β1-mediated up-regulation of p15^INK4B and p21^WAF1 transcription was abolished in TSU-Pr1 and V12Ha-Ras-transfected DU145, inhibition of the Ras/MAPK pathway restored the TGF-β1 induction of these genes. Taken together, our data suggest that prostate carcinomas with the Ras/MAPK pathway activation might have a selective growth advantage by autocrine TGF-β1 production.