To investigate the effectiveness and mechanism of apelin against pulmonary hypertension and pulmonary vascular remodeling induced by hypoxia in rats, 24 male Sprague-Dawley rats were randomly divided into normal control (NC) group, 4-week hypoxia (HH) group and 4-week hypoxia with apelin (HA) group (each n=8). The rats of hypoxic group were placed in an isobaric hypoxic chamber, in which O₂ and CO₂ content was maintained at 9%-11% and <3%, respectively, for 4 weeks (8 h/d, 6 d/week). [pGlu]apelin-13 (10 nmol/kg per day, 28 d) was administered subcutaneously by osmotic mini-pump before hypoxia treatment in HA group. L-arginine (L-Arg) uptake of pulmonary artery was assay by [³H]-L-Arg, while nitric oxide synthase (NOS) activity of pulmonary tissue, and nitrate/nitrite (NO₂(-)/NO₃(-)) concentrations in pulmonary tissue and plasma were detected by colorimetric technique and nitrate reductase method, respectively. The results showed that mean pulmonary arterial pressure, the ratio value of right ventricle weight to left ventricle plus septum weight, the relative medial thickness, and the relative medial area of pulmonary arterioles were higher in HH group than those in NC group (all P<0.01), while these indices were lower in HA group than those in HH group (P<0.05 or P<0.01). Compared with those in HH group, the uptake of 0.5, 5 and 10 nmol/L [³H]-L-Arg in pulmonary artery in HA group increased by 121.4% (P<0.01), 85.0% (P<0.05) and 61.5% (P<0.05), respectively; cNOS activity of pulmonary tissue increased by 74.3%, while iNOS activity decreased by 25.0% (all P<0.01); and NO₂(-)/NO₃(-) concentrations in pulmonary tissue and plasma increased by 97.6% and 48.0% (all P<0.05), respectively. Taken together, our results suggest that apelin has a prophylactic effect against hypoxic pulmonary hypertension in rats, and that the mechanism of this effect is possibly associated with activation of the L-Arg/NOS /NO pathway.