Brain serotonin and CRH systems participate in the control of blood glucose levels. We have previously demonstrated that the pharmacological stimulation of central 5-HT3 receptors, the target for several therapeutic agents used as antiemetics in the course of chemotherapy, induces hyperglycemia. The aim of the present study was to investigate the participation of the brain CRH component and 5-HT3 receptors in basal blood glucose levels as well as in the hyperglycemia induced by third ventricle injections of fluoxetine, a serotonin reuptake inhibitor with a broad range of clinical use. In this study, we used fasted adult Wistar male rats (220 +/- 20 g) whose third ventricles were cannulated 7 days prior to the experiments. Acute third ventricle injections of fluoxetine caused a significant increase in plasma glucose levels throughout the experiment. Pretreatment with alpha-helical CRH, a selective CRH antagonist, significantly blunted fluoxetine-induced hyperglycemia. Also, pretreatment with two distinct selective 5-HT3 receptor antagonists (LY-278,584 and ondansetron) significantly impaired the rise in plasma glucose levels observed in fluoxetine-treated animals pretreated with isotonic saline solution. None of these antagonists was able to modify blood glucose levels when injected alone into the third ventricle. Animals receiving third ventricle injections of fluoxetine, in spite of being hyperglycemic, presented plasma insulin levels similar to those displayed by normoglycemic, saline-treated controls. It is suggested that the acute increase in brain serotonergic activity caused by third ventricle injections of fluoxetine induces a hyperglycemic response that requires the functional integrity of the brain CRH system and 5-HT3 receptors. Also, it is proposed that the absence of a compensatory increase in plasma insulin levels may contribute to the generation of a hyperglycemic response after central fluoxetine administration.