Chronic unpredictable stressors can produce a situation similar to clinical depression and such animal models can be used for the preclinical evaluation of antidepressants. Nitric oxide, a secondary messenger molecule, has been implicated in neurotransmission, synaptic plasticity, learning, aggression and depression. Vaccinium myrtillus (bilberry) extract is a potent inhibitor of reactive oxygen/nitrogen species and cytokine production. The present study investigated the role of nitric oxide in the antidepressant action of Vaccinium myrtillus in unpredictable chronic mild stress-induced depression in mice. Animals were subjected to different stress paradigms daily for a period of 21 days to induce depressive-like behavior. Pretreatment with L-arginine significantly reversed the protective effect of bilberry (500 mg/kg) on chronic stress-induced behavioral (immobility period, sucrose preference) and biochemical (lipid peroxidation and nitrite levels; endogenous antioxidant activities) in stressed mice. Furthermore, L-NAME (10 mg/kg) pretreatment with a sub-effective dose of bilberry (250 mg/kg) significantly potentiated the protective effect of bilberry extract. The study revealed that modulation of the nitric oxide pathway might be involved in antidepressant-like effects of Vaccinium myrtillus in stressed mice. Copyright © 2011 John Wiley & Sons, Ltd.