The present study aimed to determine the role of arginase (Arg) in pulmonary arterial hypertension (PAH). In vitro, human pulmonary artery smooth muscle cells (HPASMCs) were cultured under hypoxic conditions with, or without, the Arg inhibitor, S‑(2‑boronoethyl)‑l‑cysteine (BEC), for 48 h, following which the proliferation of the HPASMCs was determined using MTT and cell counting assays. For the in vivo investigation, 30 male rats were randomly divided into the following three groups (n=10 per group): i) control group, ii) PAH group and iii) BEC group, in which the right ventricle systolic pressure (RVSP) of the rats was assessed. The levels of cyclin D1, cyclin‑dependent kinase (CDK)4 and p27 were measured in vitro and in vivo. The phosphorylation levels of Akt and extracellular‑related kinase (ERK) were also measured in HPASMCs. In vitro, compared with the hypoxia group, Arg inhibition reduced HPASMC proliferation and reduced the expression levels of cyclin D1, CDK4, phosphorylated (p‑)Akt and p‑ERK. By contrast, Arg inhibition increased the expression of p27. In vivo, compared with the control group, the expression levels of cyclin D1 and CDK4 were reduced in the PAH group, however, the expression of p27 and the RVSP increased. In the BEC group, the opposite effects were observed. Therefore, it was suggested that Arg inhibition may reduce the RVSP of PAH rats and reduce HPASMC proliferation by decreasing the expression levels of cyclin D1 and CDK4, increasing the expression of p27, and partly reducing the phosphorylation of Akt and ERK.