Eight highly trained cyclists were studied during exercise after glycogen depletion (test A) and during carbohydrate (CHO) loading (test B). In test B subjects were able to complete 2 h of exercise at 70-75% maximal workload (Wmax), whereas the initial intensity of 70% Wmax had to be reduced to 50% in test A. Plasma ammonia increased more rapidly, and plasma alanine, glutamate, and glutamine were lower in test A. Exercise caused a 3.6-fold increase in the proportion of active branched-chain 2-oxoacid dehydrogenase (BC) complex in muscle in test A. No activation occurred in test B. There was an inverse correlation between the activity of the BC complex and the glycogen content of the postexercise biopsies. Exercise did not cause changes in the muscle content of ATP, ADP, AMP, IMP, hypoxanthine, and lactate. It is concluded that CHO loading abolishes increases in branched-chain amino acid (BCAA) oxidation during exercise and that part of the ammonia production during prolonged exercise originates from deamination of amino acids. The data appear to confirm the hypothesis (A.J. M. Wagenmakers, J.H. Coakley, and R.H.T. Edwards. Int. J. Sports Med. 11: S101-S113, 1990) that acceleration of the BCAA aminotransferase reaction may drain the tricarboxylic acid cycle and that glycogen is a carbon chain precursor of tricarboxylic acid cycle intermediates and glutamine.