Hepatocyte growth factor (HGF) is a potent mitogen for a variety of cells including hepatocytes. While rat oval cells are supposed to be one of hepatic stem cells, biological effects of HGF on oval cells and their relevant signal transduction pathways remain to be determined. We sought to investigate them on OC/CDE22 rat oval cells, which are established from the liver of rats fed a choline-deficient/DL-ethionine-supplemented diet. The oval cells were cultured on fibronectin-coated dishes and stimulated with recombinant HGF, transforming growth factor-α (TGF-α), and thrombopoietin (TPO) under the serum-free medium condition. HGF treatment enhanced [³H]thymidine incorporation into oval cells in a dose-dependent manner. On the contrary, treatment with TGF-α or TPO had no significant effects on [³H]thymidine incorporation into the oval cells. c-Met protein was phosphorylated at the tyrosine residues after the HGF treatment. AKT, extracellular signal-regulated kinase 1/2 (ERK1/2), and p70^s6k were simultaneously activated after the HGF stimulation, peaking at 30min after the treatment. The activation of AKT, p70^s6k, and ERK1/2 induced by HGF was abolished by pre-treatment with LY294002, a phosphoinositide 3-OH kinase (PI3K) inhibitor, and U0126, a mitogen-activated protein kinase/ERK kinase (MEK) inhibitor, respectively. When the cells were pre-treated with LY294002 prior to the HGF stimulation, the proliferative action of HGF was completely abrogated, implying that the PI3K/AKT signaling pathway is responsible for the biological effect of HGF. These in vitro data indicate that HGF exerts a proliferative action on hepatic oval cells via activation of the PI3K/AKT signaling pathway.