The effects of systemic administration of fluvoxamine on extracellular serotonin (5-hydroxytryptamine, 5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) concentrations in the frontal cortex and raphe nuclei of freely moving rats were examined. Fluvoxamine significantly increased extracellular 5-HT concentrations in both regions at the two doses used (1 and 10 mg/kg i.p.). However, the increase in the raphe nuclei was several-fold that in the frontal cortex. Dialysate 5-HIAA concentrations decreased after treatment with fluvoxamine. These results confirm that 5-HT uptake inhibitors preferentially increase extracellular concentrations of 5-HT in the vicinity of cell bodies and dendrites of serotonergic neurones.