Spinal processing of somatosensory and viscerosensory information is greatly facilitated in some persistent pain states. Growing evidence suggests that the so-called central sensitization depends in part on intracellular activation and signalling via specific MAP kinases. Here we studied the expression of phosphorylated extracellular signal-regulated kinases 1 and 2 (phosphoERK), the active form of these kinases, in spinal neurons following innocuous and noxious distension of non-inflamed and cyclophosphamide (CYP)-inflamed rat urinary bladders. Additionally, we investigated the nature of bladder primary afferents responsible for spinal ERK activation. Finally, we used a specific inhibitor of ERK phosphorylation to study the influence of these kinases on the bladder reflex activity of normal and inflamed bladders. Results indicated that, in non-inflamed rats, noxious but not innocuous bladder distension significantly increased spinal phosphoERK immunoreactivity from its normal very low level. However, in CYP-inflamed rats, innocuous and noxious bladder distension significantly increased the number of spinal neurons immunoreactive to phosphoERK. ERK activation was rapid (within minutes) and transient. Desensitization of vanilloid-sensitive afferents by intravesical resiniferatoxin, a capsaicin analogue, did not decrease phosphoERK immunoreactivity in normal or CYP-inflamed rats. ERK inhibition by intrathecal PD 98059 had no effect on bladder reflex contractions of non-inflamed bladders but significantly decreased its frequency in inflamed animals. Our results suggest that spinal ERK intervene in acute and chronic inflammatory pain perception and mediate bladder reflex overactivity accompanying chronic bladder inflammation. In addition, bladder noxious input conveyed in vanilloid-resistant primary afferents is important to spinal ERK phosphorylation in both noninflamed and CYP-inflamed animals.