The effects of hypoxia on the bovine pulmonary arterial smooth muscle cell (PASM) proliferation and serotonin transporter gene expression were investigated using 3H-Thymidine incorporation, immunocytochemical staining and mRNA dot blot hybridyzation, the results were as follows, PASM 3H-Thymidine incorporation increased in response to anoxia for 24 h (P < 0.05). The anoxia-induced increased in DNA synthesis of PASM was significantly stimulated by 5-HT (100, 10 and 1 mumol/L) (P < 0.001), but 5-HT did not increase 3H-TdR incorporation under normoxic condition. Immunocytochemical reaction of serotonin in PASM under normoxic condition was markedly stronger than that of anoxic PASM (P < 0.001). Dot blot analysis showed hypoxia increased serotonin transporter mRNA expression of PASM (P < 0.01). These results indicate that stimulation of PASM proliferation with enhanced serotonin transporter gene expression and high consumption of 5-HT in hypoxia or/and anoxia may take part in the development of hypoxic pulmonany hypertention.