There are a number of studies on the effects of different NO donors and inhibitors on spinal cord with quite contradictory results. The aim of this study was to investigate the effects of sodium nitroprusside (SNP), a NO donor, and N(G)-nitro-L-arginine methyl ester (L-NAME), a nonselective NOS inhibitor, on monosynaptic and polysynaptic spinal reflexes in anesthetized and spinalized cats. After a dorsal laminectomy between L5 and S1, monosynaptic and polysynaptic spinal reflexes were evoked by stimulation of gastrocnemius nerves. Following control recordings, administration of L-NAME in 100, 200, 500 microM (local) and 10, 20, 50 mg/kg (i.v.) doses decreased significantly the monosynaptic and polysynaptic reflex amplitudes in a dose-dependent manner. Administration of SNP in 100, 200, 500 microM (local) and 100, 200, 500 microg/kg (i.v.) doses enhanced significantly the both reflex amplitudes in a dose-dependent manner. In another series of experiments it has been observed that the maximal decrease in reflex amplitudes caused by 500 microM local L-NAME administration in the 15th minute was reversed by locally administered SNP (500 microM). Our results support the hypothesis stating that NO may play a role in the modulation of mono- and polysynaptic spinal reflexes and the NO appears to have an enhancing role on these responses.