The purpose of this study was to investigate the effect of L-citrulline ingestion on ECG QT interval. To accomplish this purpose, nine male subjects (age: 23.4±0.5 years, weight: 57.7±5.6 kg) participated, who had no history of cardiovascular or respiratory function disorders. Each subject performed two tests (L-citrulline and placebo ingestion) on a separate day. ECG was taken before, and 60 min and 90 min after L-citrulline ingestion. The heart-rate-corrected QT interval (QTc) decreased significantly 60 min ( p =0.0004) and 90 min ( p =0.011) after L-citrulline ingestion. However, there were no significant changes after placebo ingestion. In addition, the interaction between L-citrulline ingestion and placebo was significant (two-way ANOVA: interaction, p =0.010). Our major finding was that L-citrulline ingestion decreases the QT interval in healthy subjects. This result suggests that transient L-citrulline ingestion may shorten the time required to complete myocardial depolarization and repolarization.