The aim of this study was to assess extracellular glucose changes in hippocampus in response to physical exercise and to determine the influence of glucose supplementation. In the same time, we have observed the changes in serotonin, in order to study the relationship between glucose and serotonin during exercise. Both glucose and serotonin were assessed using microdialysis. Exercise induced an increase in extracellular glucose levels over baseline during exercise to 121.1 +/- 3.0% (P < 0.001), then a decrease to baseline during recovery. The serotonin followed glucose changes during the first 90 min of exercise, but followed a different pattern during recovery, increasing to a maximum of 129.9 +/- 7.0% after 30 min of recovery (P < 0.001). When a 15% glucose solution was infused (10 microL x min(-1)) during exercise and recovery, blood glucose concentration was increased, but extracellular brain glucose decreased to reach a minimum of 73.3 +/- 4.6% after 90 min of recovery (P < 0.001). Serotonin was always the mirror-reflect of cerebral glucose, with a maximum increase of 142.0 +/- 6.9% after 90 min of recovery (P < 0.001). These results show that exercise induces changes in brain glucose and 5-hydroxytryptamine (5-HT) levels, which were dramatically modified by glucose infusion. Taking into account the implication of brain 5-HT in central fatigue, they suggest that if glucose supplementation, before and during exercise, undoubtedly increase performance because of its peripheral positive action, it would have a negative impact on the quality of recovery after the end of the exercise.