The participation of nitric oxide (NO) in antinociceptive activity of molsidomine and sodium nitroprusside (SNP) was studied in mice using the writhing test. Molsidomine (300 and 150 mg/kg) and SNP (1.52-0.38 mg/kg) induced antinociception that was antagonized by naloxone. L-arginine (500-62.5 mg/kg) did not produce antinociceptive effects, whereas N omega-nitro-L-arginine methyl ester (L-NAME) (37.5-150 mg/kg) induced antinociception which was suppressed by naloxone. Methylene blue did not change the molsidomine- and SNP-induced antinociception, but significantly intensified that produced by L-NAME. L-arginine increased antinociceptive effect of molsidomine but not that of SNP. Antinociceptive activity of L-NAME was partially reversed by L-arginine. D-arginine failed to influence these effects. The present findings suggest that the NO-cGMP pathway is not involved in the mechanism of molsidomine- and SNP-induced antinociception in the writhing test in mice.