Stressful lift events contribute to the development of many neuropsychiatric disorders including depression and anxiety. Animal studies based on the relationship of stress and depression or anxiety are scarce and controversial, Moreover, neither the neurobiological basis of anxiety and depression nor the mechanisms responsible for neurochemical regulation by stressful stimuli are well understood. This study was designed to investigate the possible contribution of both acute (2 h) and chronic (2 h x 15 d) restraint stress in the generation of anxiety and depression. and also to find out whether nitric oxide (NO) has a modulatory role in these behavioral reactions. Elevated plus-maze and forced swimming test (FST) were chosen for assessment of anxiety and depression, respectively. and N-G-nitro L-arginine methyl ester (L-NAME, 10 mg/kg), a NO synthase (NOS) inhibitor, and L-arginine (50 mg/kg), a NO precursor, were used to evaluate the role of nitrergic system in restraint exposed rats. The results showed that acute and chronic stress caused depression-like and anxiety-like behaviors in rats and the acute inhibition of NOS by L-NAME prevented these acute and chronic stress-induced anxiogenesis and depression. These data lead to the conclusion that stress and NO seem to be involved in the generation of anxiety and depression. (c) 2006 Prous Science. All rights reserved.