It has been proposed that extracellular ATP may be involved in visceral mechanosensory transduction by activating ligand-gated ion channels (P2X receptors). In this study, we have investigated the effects of the P2X(3) agonist alpha,beta-methylene ATP (alpha,beta-meATP) and antagonist 2',3'-O-trinitrophenyl-ATP (TNP-ATP) on pelvic afferents innervating the urinary bladder using an in vitro mouse bladder-pelvic nerve preparation. Intravesical application of alpha,beta-meATP (0.03-1 mM) increased multifibre discharges in a concentration-dependent manner. The agonist potentiated, whereas TNP-ATP (0.03 mM) attenuated, the multifibre responses to bladder distensions. Single-unit analysis revealed that both high threshold (HT) fibres (> 15 mmHg; known to be associated with nociception) and low threshold (LT) fibres (< 15 mmHg; probably associated with non-nociceptive events) could be induced to discharge by intravesical alpha,beta-meATP (1 mM, 0.1 ml). The response of the vast majority (21/22, 95.5 %) of HT fibres to bladder distensions was enhanced with a significantly reduced threshold and an increased peak response after exposure to the agonist. On the other hand, 59.7 % (46/77) of LT fibres showed a greater peak and a slightly reduced threshold for response to bladder distension in the presence of alpha,beta-meATP. An additional 11 'silent' fibres became mechanosensitive after exposure to alpha,beta-meATP. TNP-ATP (0.03 mM) did not affect the threshold of LT fibres, but it reduced the peak response of some (22/51, 43.1 %) LT fibres. Conversely, the antagonist resulted in a markedly elevated threshold and reduced peak activity in the majority (13/16, 81.3 %) of HT fibres. The results support the view that P2X(3) receptor-mediated mechanisms contribute to both nociceptive and non-nociceptive (physiological) mechanosensory transduction in the urinary bladder.