The aim of the study was (1) to determine changes in the total (TC), free (FC) and esterified (EC) L-carnitine (C) in plasma and urine of biathlonists ingesting various doses of C and (2) to find out whether the quantities of C ingested with the diet covered the metabolic needs for this compound. Male (n=24) and female (n=22) athletes, aged 17 years were divided in 4 groups given 0, 250, 500 and 750 mg . d^-1 of C, respectively. The contents of energy, selected nutrients and of C were assayed by using the food composition tables. Relative C deficiency was determined from FC/TC and EC/FC ratios obtained from plasma measurements. The supplementation with C increased its concentration in plasma and excretion of FC and TC with urine, the levels of EC remaining relatively stable. Mean values of FC/TC and EC/FC ratios were within normal limits and did not depend on the degree of supplementation with C. However, relative C deficiency was observed in two men and five women and could have resulted from a too low C intake with diet or from an intensified catabolism of fatty acids. Supplementing athletes with small doses of C (250-500 mg . d^-1) may thus be recommendable.