In vivo dialysis coupled to high-performance liquid chromatography with electrochemical detection (HPLC-EC) was used for measurement of extracellular serotonin (5-HT) in the hypothalamus of unanesthetized, unrestrained rats. A series of experiments was carried out to determine if 5-HT in the dialysis solution was released from nerve terminals. Fenfluramine, a 5-HT-releasing drug and fluoxetine, a 5-HT-reuptake inhibitor, both significantly increased extracellular 5-HT. Elevating potassium concentration in the dialysis solution also significantly increased 5-HT. Reciprocally, 5-HT was significantly reduced to about half of control levels with either local administration of tetrodotoxin, zero calcium dialysis solution, or systemic administration of 8-hydroxy-2-(di-n-propylamino)tetralin, a 5-HT1A agonist that suppresses 5-HT neuronal activity via activation of the somatodendritic autoreceptor. In addition, 5-HT levels were elevated during the dark portion of the light-dark cycle, a period when rats are more active. Changes in extracellular 5-hydroxyindoleacetic acid (5-HIAA) rarely followed changes in 5-HT. The results indicate that 5-HT in the dialysis solution, but not 5-HIAA, was a reliable measure of depolarization-induced release of 5-HT from nerve terminals. This is the first report establishing the reliability of in vivo dialysis coupled to HPLC-EC for measurement of synaptically released 5-HT.