Prostate cancer is the most commonly diagnosed neoplasm in men. LNCaP cells continue to possess many of the molecular characteristics of in situ prostate cancer. These cells lack ras mutations, and mitogen-activated protein kinase (MAPK) is not extensively phosphorylated in these cells. To determine the effects of ras/raf/MAPK pathway activation in these cells, we transfected LNCaP cells with an activatable form of c-raf-1(deltaRaf-1:ER). Activation of deltaRaf-1:ER, with resultant MAPK activation, reduced plating efficiency and soft agarose cloning efficiency 30-fold in LNCaP cells. Cell cycle distribution showed an accumulation of cells in G1 and was associated with the induction of CDK inhibitor p21WAF1/CIP1 at the protein and mRNA levels. p21WAF1/CIP1 mRNA stability was increased after deltaRaf-1:ER activation. In addition, activated deltaRaf-1:ER induced the senescence associated-beta-galactosidase in LNCaP cells. These data demonstrate that raf activation can activate growth inhibitory pathways leading to growth suppression in prostate carcinoma cells and also suggest that raf/MEK/MAPK pathway activation, rather than inhibition, may be a therapeutic target for some human prostate cancer cells.