NG-Nitro-L-arginine methyl ester (50 mg/kg s.c.), an inhibitor of nitric oxide (NO) synthase, has been reported to increase brain serotonin (5-hhydroxytryptamine, 5-HT) metabolism and induce hypophagia. Conversely, enhanced NO synthase activity is found to be accompanied by a decrease in 5-HT level. This negative correlation between NO and 5-HT in the regulation of food intake was further studied in mice. 5-HT depletion by p-chlorophenylalanine (250 mg/kg i.p., twice daily for 2 days) failed to antagonize the hypophagic effect of NG-nitro-L-arginine methyl ester. Similarly, treatment with the NO synthesis precursor, L-arginine (1000 mg/kg s.c.), did not reverse the anorexia induced by fenfluramine (10 mg/kg s.c.), a 5-HT releaser/uptake inhibitor. Pretreatment with (-)-pindolol, methylsergide and ritanserin had no effect on the hypophagic action of NG-nitro-L-arginine methyl ester, suggesting the lack of involvement of 5-HT1 and 5-HT2 receptors. The selective neuronal NO synthase inhibitor, 7-nitroindazole (12.5-50.0 mg/kg i.p.), however, did not exhibit any hypophagic effect whilst NG-nitro-L-arginine methyl ester increased gastric retention, which may subsequently induce satiety. Moreover, the hypophagic effect of NG-nitro-L-arginine methyl ester, which was unassociated with changes in water intake and malaise induction, was also unattenuated by cholecystokinin (CCK) receptor antagonists, devazepide (10 mg/kg i.p.) and PD 135,158 ([1S-[1 alpha,2 beta[S*(S*)],4 alpha ]]-4-[[2-[[3-(1 H-indol-3-yl)-2-methyl-1-oxo-2-[[[(1,7,7- trimethylbicyclo[2.2.1]hept-2-yl)oxy]carbonyl]amino]propyl]amino] -1-phenylethyl] amino]-4-oxo-butanoic acid N-methyl-D-glucamine salt; 1 mg/kg i.p.).(ABSTRACT TRUNCATED AT 250 WORDS)