Selective serotonin reuptake inhibitors (SSRIs) regulate brain noradrenergic neurotransmission both at somatodendritic and nerve terminal areas. Previous studies have demonstrated that noradrenaline (NA) reuptake inhibitors are able to desensitize α 2 -adrenoceptor-mediated responses. The present study was undertaken to elucidate the effects of repeated treatment with the SSRI citalopram on the α 2 -adrenoceptor sensitivity in locus coeruleus (LC) and prefrontal cortex (PFC), by using in vivo microdialysis and electrophysiological techniques, and in vitro stimulation of [35S]GTPγS binding autoradiography. Repeated, but not acute, treatment with citalopram (5 mg/kg, i.p., 14 days) increased extracellular NA concentration selectively in PFC. The α 2 -adrenoceptor agonist clonidine (0.3 mg/kg, i.p.), administered to saline-treated animals (1 ml/kg i.p., 14 days) induced NA decrease in LC (E max = −44 ± 4%; p < 0.001) and in PFC (E max = −61 ± 5%, p < 0.001). In citalopram chronically-treated rats, clonidine administration exerted a lower decrease of NA (E max = −25 ± 7%; p < 0.001) in PFC whereas the effect in LC was not different to controls (E max = −36 ± 4%). Clonidine administration (0.625–20 μg/kg, i.v.) evoked a dose-dependent decrease of the firing activity of LC noradrenergic neurons in both citalopram- (ED 50 = 3.2 ± 0.4 μg/kg) and saline-treated groups (ED 50 = 2.6 ± 0.5 μg/kg). No significant differences between groups were found in ED 50 values. The α 2 -adrenoceptor agonist UK14304 stimulated specific [35S]GTPγS binding in brain sections containing LC (144 ± 14%) and PFC (194 ± 32%) of saline-treated animals. In citalopram-treated animals, this increase did not differ from controls in LC (146 ± 22%) but was lower in PFC (141 ± 8%; p < 0.05). Taken together, long-term citalopram treatment induces a desensitization of α 2 -adrenoceptors acting as axon terminal autoreceptors in PFC without changes in somatodendritic α 2 -adrenoceptor sensitivity.