We studied nitric oxide (NO) synthase activity and serotonin content in the diencephalon of 24 hr food deprived rats. NO synthase activity was significantly increased whereas serotonin levels together with those of tryptophan and 5-hydroxyindoleacetic acid (5-HIAA) were reduced in food deprived rats when compared to control rats. NG-Nitro-L-arginine (L-NO Arg), an inhibitor of NO synthase, was used as a tool to study the role of NO in food deprivation. Twenty-four hr food deprived male Sprague-Dawley rats were intraperitoneally (i.p.) administered L-NO Arg (12.5, 25 and 50 mg/kg) before food presentation. Control rats received a NaCl (0.9%) solution. Food consumption was monitored 1 and 2 hr after food presentation. L-NO Arg administration produced a dose-dependent reduction in food intake. Pretreatment with metergoline (2 mg/kg) but not with ritanserin (1 mg/kg) antagonized the anorectic effect of L-NO Arg. Moreover, in the diencephalon L-NO Arg significantly reduced NO synthase activity whereas it increased serotonin levels. Our data indicate that NO might have a physiological role in the regulation of food intake and suggest that brain NO may modulate the central serotoninergic system.