The effect of altering hippocampal nitric oxide (NO) levels on basal and N-methyl-d-aspartate receptor-evoked release of GABA has been studied in freely moving rats. N-Methyl-d-aspartate (NMDA) increased extracellular GABA in a concentration-dependent manner. The nitric oxide synthase inhibitor t-nitro-arginine-methyl ester (l-NAME; 100 μM) increased basal GABA release, and also enhanced release of GABA evoked by NMDA (100 μM) compared with the same concentration of NMDA infused alone. 200 μM l-NAME increased basal dialysate GABA, but to a lesser extent than the 100 μM concentration of the drug, and the NMDA-induced release of GABA was decreased. 1.0 MM l-NAME significantly decreased basal extracellular GABA, while abolishing the NMDA-evoked release of the amino acid. The actions of l-NAME were not mimicked by its much less active isomer d-nitro-arginine-methyl ester. The NO donor S-nitroso-N-acetylpenicillamine decreased dialysate GABA at a 500 μM concentration but increased the extracellular level of the transmitter when infused at 1.0 and 2.0 mM concentrations. These data suggest that NO may mediate both excitatory and inhibitory functions in vivo.