baclofen a gabab receptor agonist inhibits human hepatocellular carcinoma cell growth in vitro and in vivo gamma aminobutyric acid gaba has been reported to affect cancer development but the activation of its type b receptor gababr has shown contradictory effects on the progress of human carcinoma in this study we investigated the antitumor effect of the gababr agonist baclofen bac on growth of human hepatocellular carcinoma hcc in vitro and in vivo we found bac induced g0g1 phase arrest which was associated with downregulation of intracellular camp level and upregulation of p21waf1 protein expression as well as its phosphorylation level these in vitro effects could be abrogated by pretreatment with the specific gababr antagonist phaclofen pha moreover systemic administration of bac significantly suppressed bel7402 xenograft tumor growth our data support the inhibitory effect of gababr activation on hcc development which would raise the possibility to develop bac as a therapeutic drug for the treatment of hcc