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H ypoxia is a well-recognized stimulus for pulmonary blood vessel remodeling and pulmonary hypertension development. One mechanism that may account for these effects is the direct action of hypoxia on the expression of specific genes involved in vascular smooth muscle cell (SMC) proliferation. Previous studies demonstrated that the serotonin (5-hydroxytryptamine; 5-HT) transporter (5-HTT) mediates the mitogenic activity of 5-HT in pulmonary vascular SMCs and is overexpressed during hypoxia. Thus, 5-HT-related mitogenic activity is increased during hypoxia. Here, we report that mice deficient for 5-HTT (5-HTT^–/–) developed less hypoxic pulmonary hypertension and vascular remodeling than paired 5-HTT^+/+ controls. When maintained under normoxia, 5-HTT^–/–-mutant mice had normal hemodynamic parameters, low blood 5-HT levels, deficient platelet 5-HT uptake, and unchanged blood levels of 5-hydroxyindoleacetic acid, a metabolite of 5-HT. After exposure to 10% O₂ for 2 or 5 weeks, the number and medial wall thickness of muscular pulmonary vessels were reduced in hypoxic 5-HTT^–/– mice as compared with wild-type paired controls. Concomitantly, right ventricular systolic pressure was lower and right ventricle hypertrophy less marked in the mutant mice. This occurred despite potentiation of acute hypoxic pulmonary vasoconstriction in the 5-HTT^–/– mice. These data further support a key role of 5-HTT in hypoxia-induced pulmonary vascular SMC proliferation and pulmonary hypertension.