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We investigated the effect of nitric oxide (NO) on N-methyl-D-aspartate (NMDA)-induced changes in levels of dopamine (DA) and its metabolite in the striatum using in vivomicrodialysis. Local administration of 1 mM NMDA into the striatum significantly augmented DA release in the striatum. Simultaneous administration of 5 mM NG-nitro-L-arginine methyl ester (-NAME), a NO synthase inhibitor, into the striatum significantly potentiated NMDA-induced DA release. This effect of L-NAME was completely reversed in the presence of 50 mM L-arginine (L-Arg). Administration of 1 mM NMDA significantly decreased the levels of dihydrox-yphenylacetic acid (DOPAC) and homovanillic acid (HVA). This effect of NMDA was not affected by concurrent administration of L-NAME. This study provides in vivoevidence for the involvement of NO in NMDA-induced DA release.