Several studies of the relationship of food intake to physical exercise strongly suggest that the two are closely linked. Serotonin transmission seems to be particularly important in the regulation of both appetite and central fatigue. Thus, the ingestion of branched-chain amino acids has been reported to decrease serotonin production and to improve physical performance. In the present study, the effects of three treatments (water, glucose, and branched-chain amino acids) were compared in 34 male Wistar rats. Maximal exercise duration, blood insulin, and glucose levels were measured. Results showed that following the ingestion of branched-chain amino acids, physical performance is lower and blood insulin level is higher than after glucose administration.