Aim:To investigate the effect of 5-hydroxytryptamine transporter (5-HTT) inhibitor fluoxetine and antisense oligodeoxynucleotide (ODN) to extracelluar signal regulated kinases (ERKs) on pulmonary arterial smooth muscle cells (PASMCs) proliferation induced by 5-HT. Methods: Liposomal transfection was used to introduce ODNs to ERK1/2 into cultured rat PASMCs and the transfection effi ciency was measured by observing the uptake of the fluorecein isothiocynate (FITC)-labeled antisense ODN in PASMCs. The effects of 5-HTT selective inhibi tor fluoxetine and ODNs on the proliferation of PASMCs were evaluated by cell number counting and cell cycle analysis, and measured by microculture tetrazo lium (MTT) assay and flow cytometry (FCM), respectively. Results: Liposomes mediated the transfection of ODNs into PASMCs with high efficiency. MTT assay showed fluoxetine (10 μmol/L, 1 μmol/L, and 100 nmol/L) concentration dependently inhibited the proliferation of PASMCs induced by 5-HT (1 μmol/L) in vitro. The proliferation rate of PASMCs by 5-HT was significantly inhibited by pretreatment with ERK1/2 antisense ODN (0.2 μmol/L) from 251%± 18% to 86%±5% (P＜0.01). Flow cytometric analysis of cell cycle distribution showed that the increase of 5-HT induced S phase fraction (SPF) and proliferation index (PI) were significantly inhibited by fluoxetine (1 μmol/L) or antisense ODN with SPF from 36%±4% to 26%±3% and 24%±4%, and PI from 34%±2% to 29%±2% and 24%±2%,respectively. Conclusion: 5-HTT mediates the mitogenic effect of 5-HT on PASMCs and the proliferation of PASMCs induced by 5-HT is dependent on ERKs signal pathway.