This study measured release of glutamate (GLU) and citrulline (CIT), an amino acid co-product of nitric oxide synthesis in response to spinal administration of NMDA. Rats were implanted with intrathecal (IT) catheters and microdialysis probes. Intrathecal NMDA evoked release of GLU and CIT (mean peak response 207 +/- 36 s.e.m. and 223 +/- 50% baseline, respectively). IT pretreatment with L-NAME (mean peak response 104 +/- 16 and 121 +/- 5% baseline, respectively), but not D-NAME, blocked evoked release of both GLU and CIT. Thus, activation of NMDA receptors results in extracellular release of GLU and nitric oxide. Inhibition of nitric oxide synthase blocked evoked increases in extracellular GLU suggesting that augmented GLU release occurs secondary to nitric oxide production.