Asymmetric dimethylarginine (ADMA) is a risk factor for endothelial dysfunction. The polypeptide apelin has biphasic effects on blood vessels in vivo and in vitro. We investigated the effect of apelin-13 on ADMA-damaged vessels. Rats were divided among ADMA-treated and control groups, which were treated with ADMA (10 mg.(kg body mass)(-1).day(-1)) or saline, respectively, for 4 weeks. Systolic blood pressure (SBP) was measured before and after the injection of apelin-13. The ultrastructure of endothelial cells in caudal arteries was examined using transmission electron microscopy. The reactivities of isolated caudal artery rings were observed after exposure to apelin-13, and myosin light chain (MLC) phosphorylation was assessed by immunohistochemistry in rings treated with or without apelin-13. ADMA induced hypertension and endothelial dysfunction. After injection of apelin-13, SBP declined in the control group but was elevated in the ADMA-treated group. In vitro, apelin-13 caused relaxation in rings in the control group, but it contracted rings in the ADMA-treated group. Apelin-13 promoted MLC phosphorylation in vascular smooth muscle cells (VSMCs) in the ADMA group. These results indicate that apelin-13 might pass through ADMA-damaged endothelium and act on VSMCs to increase MLC phosphorylation, thus contributing to vasoconstriction and exacerbating hypertension.