Recently identified suppressors of cytokine signaling (SOCS) have been proposed as negative regulators of cytokine signaling, which have distinct mechanisms of inhibiting JAK-STAT pathway. In this study, using cultures of rat primary pulmonary vascular smooth muscle cells (PASMC), we found that hypoxia induced strongly STAT3 phosphorylation by up to four-fold. At the same time, mRNA for the endogenous cytokine signaling repressor SOCS3, but not SOCS1, was markedly induced in PASMC as early as 2h following hypoxic stimulation. Furthermore, forced expression of SOCS3 gene suppressed tyrosine phosphorylation of STAT3 and transcription of c-myc gene by more than 70% and 60% in PASMC under hypoxic conditions, respectively. Additionally, we showed here that hypoxia enhanced nearly two-fold increase of PASMC proliferation and overexpression of SOCS3 gene downregulated hypoxia-induced PASMC proliferation by about 50%. The finding suggest that STAT3-dependent pathway is involved in the activation and proliferation of PASMC stimulated by hypoxia, and SOCS3 is a rapidly hypoxia-inducible gene and acts to inhibit activation of cellular signaling pathway in a classical negative feedback loop.