Abstract. Valproate (Vpa) is a well-characterized histone deacetylase inhibitor with anti-neoplastic properties. We analyzed the growth blocking effects and the molecular mode of action of this compound in colorectal cancer cells in vitro and in vivo. caco-2, SW-480, cx-1 or Widr cell lines were exposed to Vpa (0.25-2 mm) for various time periods. cell growth, cell cycle progression and apoptosis were analyzed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide dye reduction assay and flow cytometry. Cell cycle-and apoptosisregulating proteins and histone acetylation were assessed by Western blotting. In vivo tumor growth and regulating protein expression under Vpa were investigated in a subcutaneous xenograft tumor model. Vpa inhibited the growth of all cell lines with cell cycle arrest paralleled by the up-regulation of h3 and h4 acetylation. In vivo tumor growth was substantially depressed by Vpa (200 mg/kg bw). cell cycle proteins (cdk1, cdk2, cdk4, cyclin d, cyclin E, p19, p21 and p27) were differentially altered by Vpa. predominantly cdk1 was decreased and p27 was up-regulated in all models. apoptosis-related proteins were altered in vivo with up-regulation of bax and down-regulation of bcl-2. Vpa exerts anti-neoplastic activity in colorectal tumor cell lines in vitro and in vivo by altering cell cycle regulation.