Vascular remodeling and smooth muscle cell proliferation are hallmark pathogenic features of pulmonary artery hypertension (PAH). Alterations in the metabolism of L-arginine via arginase and nitric oxide synthase play a critical role in the endothelial dysfunction seen in PAH. L-arginine metabolism by arginase produces L-ornithine and urea. L-ornithine is a precursor for polyamine and proline synthesis, ultimately leading to an increase in cellular proliferation. Given the integral role of the smooth muscle layer in the pathogenesis of hypoxia-induced PAH, we hypothesized that hypoxia would increase cellular proliferation via arginase induction in human pulmonary artery smooth muscle cells (hPASMC). We found that arginase II mRNA and protein expression were significantly increased in cultured hPASMC exposed to 1% O₂ for 24 and 48 hours, which coincided with an increase in arginase activity at 48 hours. There were no hypoxia-induced changes in levels of arginase I mRNA or protein in cultured hPASMC. Exposure to hypoxia resulted in more than one and a half times as many viable cells after 120 hours than normoxic exposure. The addition of the arginase inhibitor, S-(2-boronoethyl)-L-cysteine, completely prevented both the hypoxia-induced increase in arginase activity and proliferation in hPASMC. Furthermore, transfection of siRNA targeting arginase II in hPASMC resulted in knock down of arginase II protein levels and complete prevention of the hypoxia-induced cellular proliferation. These data support our hypothesis that hypoxia increases proliferation of hPASMC through the induction of arginase II.