Increased activity of glutamate N-methyl-D-aspartate (NMDA) receptors is the dominant mechanism by which nitric oxide (NO ⅐ ) is generated. By using a selective direct-current amperometry method, we characterized real time NO ⅐ release in vivo in response to chemical stimulation of NMDA receptors in the rat striatum. The application of NMDA caused the appearance of a sharp and transient oxidation signal. Concentration-response studies (10 -500 M) indicated an EC 50 of 48 M. The NMDA-induced amperometric signal was suppressed by focal application of the nitric-oxide synthase inhibitor L-nitro-arginine methyl ester (L-NAME, 100 M) or D-(Ϫ)-2-amino-5-phosphonopentanoic acid (AP-5, 100 M) or by systemic injection of dizocilpine (1 mg/kg i.p.), drugs that, when given alone, had no effect on baseline oxidation current. Repeated injections of NMDA at short intervals (ϳ80 s) resulted in a progressive reduction of the amperometric signal with a decay half-life of 1.36 min. Sixty min after the last NMDA application the amperometric response was restored to initial levels. Finally, the coapplication of glycine (50 or 100 M), which, when given alone had no effect, potentiated the NMDA-induced response. Thus, NMDA receptor-mediated activation of striatal NO ⅐ system shuts off quickly and undergoes rapid desensitization, suggesting a feedback inhibition of NMDA receptor function. To the extent of NO ⅐ release can represent a correlate of NMDA receptor activity, its amperometric detection could be useful to assess in vivo the state of excitatory transmission under physiological, pharmacological, or pathological conditions. Nitric oxide (nitrogen monoxide, NO ⅐ ) is a free radical species that is released in the brain in response to neuronal depolarization. NO ⅐ is a small, lypophylic molecule that readily diffuses between and within cells. As such, it can act distally of the cellular site of production, but it rapidly decays due to its short half-life (0.2-120 s; Kelm et al., 1988;Moncada et al., 1989;Ford et al., 1993;Wood and Garthwaite, 1994;Crespi et al., 2001). In the brain NO ⅐ is implicated, among other things, in neurotransmitter release, learning and memory, neuronal plasticity, and gene expression and neuronal degeneration and survival (Snyder, 1992;Bruhwyler et al., 1993;Schuman and Madison, 1994). Although several pathways that cause nonspecific depolarizations and elevations of intracellular Ca 2ϩ can stimulate NO ⅐ formation, increased activity of glutamate N-methyl-D-aspartate (NMDA) receptor represents the dominant mechanism by which NO ⅐ is generated in the brain (Garthwaite and Garthwaite, 1987;Garthwaite and Boulton, 1995). Indeed, the NMDA receptor complex is physically coupled (Sheng and Pak, 2000) to nitric-oxide-synthase (NOS; EC 1.14.13.39), a Ca 2ϩ -calmodulin-sensitive, protein kinase C-dependent enzyme that catalyzes the formation of NO ⅐ from L-arginine. Therefore, NO ⅐ formation and release can be considered a neurochemical correlate of the functional activation of NMDA receptor-mediated...