To examine the effects of L-carnitine supplementation on short high-intensity exercise, twenty male collegiate swimmers completed two trials separated by seven days. Each trial consisted of five 91.4 m (100 yd) swims with a two minute rest interval between each bout. Following the first trial subjects were evenly and randomly assigned to either an L-carnitine (LC) group or a placebo (PL) group. The LC group ingested 2 grams L-carnitine in a citrus drink twice daily for 7 days, while the PL group received only the citrus drink during the same time period. Performance times were recorded for each repeat during both trials. Blood samples (5 ml) were obtained from an antecubital vein 1 minute following the interval set. Blood pH, base excess (BE), lactate (LA), carnitine and carnitine fractions were measured. Total serum carnitine was significantly (p < 0.05) elevated (75.9 +/- 2.0 vs. 106.4 +/- 3.5 mumol.l-1) in the LC group following treatment, while the PL group was unchanged (79.5 +/- 2.8 vs. 77.6 +/- 5.3 mumol.l-1). Free and short-chain serum carnitine fractions were also increased (p < 0.05) in the LC group, but were not altered in the PL group. No differences in performance times were observed between trials or between groups. Blood pH, LA and BE revealed a similar response in both groups during each trial. Despite the elevation in serum L-carnitine and carnitine fractions, these results indicate that L-carnitine supplementation does not provide an ergogenic benefit during repeated bouts of high-intensity anaerobic exercise in highly trained swimmers.