Serotonin (5-HT) has been reported to be involved in cancer progression by stimulating angiogenesis and cell growth. In this study， we examined the expression of the serotonin transporter (5-HTT) and the role of histone deacetylases (HDACs) in regulating the 5-HTT gene in tumor cells. The 5-HTT gene expression was almost silenced in chicken lymphoma DT40， myelomonocytic tumor HD11 and hepatoma DU249 cells， compared to their physiological counterpart. In contrast， HDAC1 mRNA expression was increased in these cell lines. Indeed， the pan-HDAC inhibitor trichostatin A (TSA) enhanced the 5-HTT mRNA expression in several tumor cell lines including the human cell lines HepG2 and THP-1 and increased the 5-HT uptake in HD11 cells. In addition， treatment with parthenolide， which is capable of depleting HDAC1， and knockdown of HDAC1 using siRNA resulted in increased 5-HTT mRNA expression， confirming the role of HDAC1 in the down-regulation of 5-HTT in the tumor cells. Deletion analysis of the 5-HTT promoter and site-directed mutagenesis revealed that the transcription factor CCAAT/enhancer binding protein beta (C/EBPß)， in interacting with the 5-HTT promoter， mediated both the inhibition of the 5-HTT expression by HDAC1 and the activation by CREB-binding protein (CBP). Using a chromatin immunoprecipitation assay， we found increased acetylation of histone H4 associated with the 5-HTT promoter in cells treated with TSA. Our results suggest that the 5-HTT gene is epigenetically downregulated by HDAC1 in several types of cancer.