Twenty nonvegetarian active males were pair-matched and randomly assigned to receive 2 g of L-carnitine L-tartrate (LC) or placebo per day for 2 wk. Participants exercised for 90 min at 70% VO2max after 2 days of a prescribed diet (M +/- SD: 13.6 +/- 1.6 MJ, 57% carbohydrate, 15% protein, 26% fat, 2% alcohol) before and after supplementation. Results indicated no change in carbohydrate oxidation, nitrogen excretion, branched-chain amino acid oxidation, or plasma urea during exercise between the beginning and end of supplementation in either group. After 2 wk of LC supplementation the plasma ammonia response to exercise tended to be suppressed (0 vs. 2 wk at 60 min exercise, 97 +/- 26 vs. 80 +/- 9, and 90 min exercise, 116 +/- 47 vs. 87 +/- 25 micromol/L), with no change in the placebo group. The data indicate that 2 wk of LC supplementation does not affect fat, carbohydrate, and protein contribution to metabolism during prolonged moderate-intensity cycling exercise. The tendency toward suppressed ammonia accumulation, however, indicates that oral LC supplementation might have the potential to reduce the metabolic stress of exercise or alter ammonia production or removal, which warrants further investigation.