Alterations in central monoaminergic neurotransmission are important in the actions of many antidepressants. This study tested the hypothesis that tonic elevation of noradrenergic (NA) neurotransmission in medial prefrontal cortex (mPFC) by chronic treatment with the selective norepinephrine (NE) reuptake blocker desipramine (DMI) may contribute to the beneficial cognitive effects of this antidepressant drug (AD). Male Sprague-Dawley rats were treated with DMI acutely (15 mg/kg, i.p.) or chronically for 21 days (7.5 mg/kg/day via osmotic minipump) before assessing performance on an attentional set-shifting test. The extradimensional set-shifting component of this test reflects a process of cognitive flexibility that is dependent upon mPFC, and that we have shown previously to be facilitated by NA activity in mPFC. Microdialysis was performed to measure NE release in mPFC concurrently with behavioral testing. Acute DMI treatment produced an increase in extracellular NE levels in mPFC, and a modest improvement in overall performance across all task stages of the attentional set-shifting test, but failed to produce a significant improvement in any of the individual specific tasks comprising the test sequence. Chronic DMI treatment tonically elevated basal extracellular NE levels in mPFC, associated with a significant improvement in performance specifically on the extradimensional set-shifting component of the test. There was also a significant reduction in set loss errors in rats treated chronically with DMI. Hence, tonic elevation of NA transmission in mPFC by chronic DMI treatment was associated with a time-dependent facilitation of cognitive flexibility that may contribute to the mechanism whereby chronic treatment with ADs, specifically NE reuptake blockers, may exert a beneficial therapeutic effect on cognition in depressed patients.