Objective To explore the effects of hypoxia on expression of signal transduction and activators of transcription (STAT3) gene and proliferation of rat pulmonary arterial smooth muscle cells (PASMCs). Methods RT PCR and western blot were used to assess the levels of STAT3 mRNA and tyrosine phosphorylated STAT3 protein after primarily cultured rat PASMCs were exposed to hypoxia for 2h, 6h, 12h, 24h respectively. 3 H TdR incorporation was used to observe the cell proliferation. Results The expression of STAT3 mRNA was increased after exposure of PASMCs to hypoxia for 2h, and peaked when exposed for 6h, then declined when exposed for 12h, but still higher than that in PASMCs when exposed to normal oxygentension. The level of tyrosine phosphorylated STAT3 was increased when exposed for 6h,peaked after 12h and delined after 24h in PASMCs exposed to hypoxia. With prolongation of time of hypoxia, 3 H TdR incorporation in PASMCs was significantly higher, and reached a maximum after 24h. Conclusion The results suggested that hypoxia could induce the expression of STAT3 mRNA and activation of STAT3 protein in PASMCs, implying that STAT3 gene might play an important role in the early stage of hypoxia induced PASMCs proliferation