Endogenous nitric oxide synthase (eNOS) inhibitor asymmetric dimethylarginine (ADMA) is a cardiovascular risk factor. We tested the hypothesis that L-citrulline may ameliorate the endothelial function altered by ADMA in porcine coronary artery (PCA). Myograph study for vasorelaxation, electrochemical measurement for NO, RT-PCR, and Western blot analysis for expression of eNOS, argininosuccinate synthetase (ASS), and p-eNOS(ser1177) were performed. cGMP was determined by enzyme immunoassay. Superoxide anion (O2.(-)) production was detected by the lucigenin-enhanced chemiluminescence method. Compare with controls (96.03% ± 6.2%), the maximal relaxation induced by bradykinin was significantly attenuated (61.55% ± 4.8%, p<0.01), and significantly restored by L-citrulline (82.67 ± 6.4%, p<0.05) after 24 hours of ADMA exposure. Expression of eNOS, p-eNOS(ser1177), and ASS in PCA significantly increased after L-citrulline incubation. L-citrulline also markedly restored the NO production, and cGMP level which was reduced by ADMA. The increased O2.(-) production by ADMA was also inhibited by L-citrulline. L-citrulline restores the endothelial function in preparations treated with ADMA by preservation of NO production and suppression of O2.(-) generation. Preservation of NO is attributed to the upregulation of eNOS expression along with activation of p-eNOS(ser1177). L-citrulline improves endothelium-dependent vasodilation through NO/ cGMP pathway.