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1. An increased uptake of tryptophan in the brain may increase serotoninergic activity and recently has been suggested to be a cause of fatigue during prolonged exercise. The present study, therefore, investigates whether ingestion of tryptophan or the competing branched‐chain amino acids (BCAAs) affect performance. Ten endurance‐trained male athletes were studied during cycle exercise at 70‐75% maximal power output, while ingesting, ad random and double‐blind, drinks that contained 6% sucrose (control) or 6% sucrose supplemented with (1) tryptophan (3 g l‐1), (2) a low dose of BCAA (6 g l‐1) or (3) a high dose of BCAA (18 g l‐1). 2. These treatments greatly increased the plasma concentration of the respective amino acids. Using the kinetic parameters of transport of human brain capillaries, BCAA supplements were estimated to reduce brain tryptophan uptake at exhaustion by 8‐12%, while …