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The involvement of nitric oxide (NO) in anxiety was investigated in rats, using the elevated plus maze test. Acute, but not chronic, systemic treatment with N^ω-nitro-l-arginine methyl ester (l-NAME, 10 and 60 mg·kg⁻¹), an inhibitor of NO synthase, increased the time spent by the rats in the open arms. Both the acute and chronic treatments with l-NAME inhibited NO synthase in endothelial cells and in the central nervous system, as shown by the increase in mean arterial pressure and decreased NO synthase activity in brain tissue. Chronic treatment with l-NAME also decreased the serum nitrate levels. The anxiolysis induced by acute l-NAME treatment is unlikely to be due to hypertension, since two-kidney one-clip hypertension in non-l-NAME-treated rats failed to significantly change exploratory behaviour in the elevated plus maze. These results indicate that acute inhibition of NO synthesis decreases anxiety in rats …