Dorsal horn sensitization following somatic noxious stimuli is partly mediated by the N-methyl-D-aspartate (NMDA) sub-type of glutamate receptor. This phenomenon has been comparatively sparsely investigated in the area of visceral pain. We have therefore investigated the role of spinal NMDA receptors in central sensitization in an animal model of persistent visceral pain. In anaesthetized rats the lumbosacral spinal cord was exposed by laminectomy and the pre-emptive effect of intrathecal AP-5 upon the hyper-reflexia associated with chemical inflammation of the bladder was investigated. The effect of intrathecal AP-5 (an NMDA receptor antagonist) upon the normal cystometrogram (CMG) was also measured. AP-5 (125-1000 micrograms) prevented the hyper-flexia associated with bladder inflammation in a dose-dependant fashion. In general, within the dose range 62.5-1000 micrograms, AP-5 had no significant effect upon the normal micturition reflex. However, at the top of this dose range a minor non-significant depression of this reflex was noted. NMDA receptors do not appear to mediate the micturition reflex at a spinal cord level. However, they are involved in the induction of hyper-reflexia following urinary bladder inflammation, this hyper-reflexia can be prevented by pre-emptive intrathecal administration of AP-5.