Fatigue during prolonged exercise has traditionally been attributed to the occurrence of a "metabolic end point", where muscle glycogen concentrations are depleted, plasma glucose concentrations are reduced, and plasma free fatty acid levels are elevated. But there exists also a "central fatigue hypothesis" which is based on the increase in the concentration of brain serotonin (5-Hydroxytryptamine or 5-HT) during exercise. However, the physiological mechanisms for central fatigue are largely unexplored, therefore we designed several experiments where central serotonergic activity was manipulated. These animal and human experiments showed that although brain neurotransmission had significantly increased, the supplementation with L-TRP did not lead to premature fatigue. In human studies we used several reuptake inhibitors in order to modify brain activity during exercise. These results clearly showed that time trial performance could not be influenced, but that during prolonged exercise the brain activity can be influenced, as measured by the peripheral hormones.