Drought-tolerant wild watermelon accumulates high levels of citrulline in the leaves in response to drought conditions. In this work, the hydroxyl radical-scavenging activity of citrulline was investigated in vitro. The second-order rate constant for the reaction between citrulline and hydroxyl radicals was found to be 3.9x10(9) M(-1) s(-1), demonstrating that citrulline is one of the most efficient scavengers among compatible solutes examined so far. Moreover, citrulline effectively protected DNA and an enzyme from oxidative injuries. Liquid chromatography-mass spectrometry analysis revealed that at least four major products were formed by the reaction between citrulline and hydroxyl radicals. Activities of metabolic enzymes were not inhibited by up to 600 mM citrulline, indicating that citrulline does not interfere with cellular metabolism. We reasoned, from these results, that citrulline contributes to oxidative stress tolerance under drought conditions as a novel hydroxyl radical scavenger.