L-carnitine is essential to cellular energy production mainly because of its acyl- and acetyl-carrier properties. Athletes commonly take L-carnitine, which is thought to improve exercise performance. There are no reports on carnitine plasma concentrations and carnitine excretion in short-duration maximal exercise in well-trained athletes taking this substance. We measured plasma and urine carnitine concentrations before and 10 min after maximal treadmill ergometry in nine well-trained sportsmen with and without oral supplementation with 1 g L-carnitine. In athletes without L-carnitine intake, plasma free carnitine concentration decreased significantly from 45.2 +/- 5.3 to 41.6 +/- 5.0 mumol/l (mean +/- SD, p < 0.001) 10 min after exercise compared with baseline. In athletes with oral L-carnitine supplementation, plasma free carnitine concentration at baseline was 71.3 +/- 10.2 mumol/l and did not change after maximal exercise (71.8 mumol/l +/- 10.7 mumol/l). The elevated plasma concentration of free carnitine without decrease after maximal exercise in well-trained athletes taking L-carnitine could be important in view of the newly postulated direct vascular effects of L-carnitine in improving skeletal muscle performance.