Changes in the main physiological parameters and circulating indicators of carbohydrate, protein, lipid (and ketone body) metabolism were measured in ten exercising subjects before L-carnitine (L-carn) loading, after 4 weeks of daily loading with 2 g L-carn, and 6-8 weeks after terminating L-carn administration. Measurements were made on venous blood samples collected during each experiment at fixed time intervals over an initial rest of 45 min, 60 min bicycle exercise performed near 50% VO2max and 120 min recovery. Free and total plasma carnitine levels reached a plateau corresponding to an average rise of 25% for both fractions, 9-10 days after the beginning of the L-carn diet. These levels returned to their initial values 6-8 weeks after cessation of the supply. Generally L-carn supplementation did not significantly modify the physiological parameters and circulating metabolites. No distinct increase of the relative participation of endogenous lipids in the fuel supply of prolonged submaximal exercise was observed. In normal human subjects the increased demand for fatty acid oxidation resulting from exercise seems to be adequately supported by endogenous levels of carnitine.