Serotonin (5-HT) syndrome is a potentially fatal condition associated with various combinations of serotonergic drugs. The present study was undertaken to demonstrate that nervous systems other than the 5-HT system also participate in the pathophysiology of 5-HT syndrome. Concentrations of 5-HT, dopamine (DA) and glutamate in the hypothalamus were measured in two different 5-HT syndrome animal models using a microdialysis technique. The first model was induced by tranylcypromine, a nonselective monoamine oxidase (MAO) inhibitor (3.5 mg/kg) and fluoxetine, a selective serotonin reuptake inhibitor (SSRI) (10 mg/kg). The second model was induced by clorgyline, an MAO-A inhibitor (1.2 mg/kg) and 5-hydroxy-L-tryptophan, a precursor of 5-HT (5-HTP) (80 mg/kg). In the first model, the levels of 5-HT and DA increased by 40-fold and 44-fold, respectively, compared with the preadministration levels. In the second model, the concentrations of 5-HT increased by up to 140-fold, whereas DA levels increased by only 10-fold, of the preadministration levels. Although the level of glutamate in the second model barely changed, a delayed increase in the glutamate level was observed in the first model. These findings suggest that not only hyperactivity of the 5-HT system, but also hyperactivity of the DA system, are present in 5-HT syndrome, and that the glutamatergic system is influenced in some 5-HT syndrome cases in which the DA concentration markedly increases.