The effect of L-carnitine supplementation on the plasma carnitine levels and various performance parameters of seven male marathon athletes was investigated. Each athlete completed two progressive treadmill tests to exhaustion, separated by a six week supplementation period (2g L-carnitine/day). During the treadmill tests peak treadmill running speed, oxygen consumption, cardiovascular responses, respiratory exchange ratio and plasma carnitine levels were monitored. After supplementation peak treadmill running speed increased by 5.68%, average oxygen consumption (VO 2) and heart rate decreased, and respiratory exchange ratio values showed a declining tendency. After supplementation plasma acyl-, total- and percentage bound carnitine levels increased significantly, whereas free carnitine levels increased but remained below the normal range. Findings from this study suggest that L-carnitine supplementation could positively influence aerobic capacity.