The effect of altering hippocampal nitric oxide levels on basal and N-methyl-d-aspartate (NMDA) receptor-evoked release of glutamate and aspartate has been studied in freely moving rats. NMDA increased extracellular glutamate and aspartate in a concentration-dependent manner. The nitric oxide synthase inhibitor l-nitro-arginine-methyl ester (l-NAME; 100 μM) increased basal glutamate and aspartate release, and also enhanced release of these amino acids evoked by NMDA (100 μM) compared with the same concentration of NMDA infused alone. l-NAME at 200 μM increased basal dialysate glutamate, but not aspartate, to a lesser extent than the 100 μM concentration of the drug, and the NMDA-induced release of glutamate and aspartate was decreased. l-NAME at 1.0 mM did not significantly alter basal extracellular glutamate but significantly decreased dialysate aspartate, while abolishing the NMDA-evoked release of both amino acids. The actions of l-NAME were not mimicked by its much less active isomer d-nitro-arginine-methyl ester. The nitric oxide donor drug S-nitroso-N-penicillamine decreased dialysate glutamate and aspartate at a 500 μM concentration but increased the extracellular level of both amino acids when infused at 1.0 mM and 2.0 mM concentrations. These data suggest that nitric oxide may mediate both excitatory and inhibitory functions, according to the level of nitric oxide production in vivo.