Ecklonia cava polyphenol (ECP) is a potent antioxidant and procirculatory agent that may contribute to improvement of endurance performance during highly intense exercise. This study evaluated the acute effect of an ECP-supplemented drink against a placebo on maximum endurance capacity and related physiological parameters. Twenty men 18-23 yr old volunteered as participants. Each performed 2 randomized trials with a 1-week interval between them. One trial was with ECP and the other with a placebo drink. Participants in this randomized, placebo-controlled, crossover design ingested either a placebo or ECP drink 30 min before each exercise trial. Time to exhaustion, VO(2max), and postexercise blood glucose and lactate levels were evaluated. ECP supplementation increased time to exhaustion (2.39 min) compared with placebo. This result was accompanied by a 6.5% higher mean VO(2max) in the ECP group, although the difference was not statistically significant. The blood glucose level in the ECP group at 3 min after exhaustive exercise was significantly higher than that of the placebo group (+ 9.9%). The postexercise blood lactate levels in the ECP group showed a decreasing trend compared with placebo, but it was nonsignificant. This study was not able to determine any physiological mechanisms behind the improved endurance performance, but, based on these results, it is speculated that the ECP supplementation may have contributed to enhanced oxidation of glucose and less production of lactate during intense exercise, possibly by its free-radical-scavenging and procirculatory activities. However, careful verification is required to elucidate the correct mechanism.