We have shown the presence and activity of ATP-gated ion channels (P2X receptors) in nociceptive nerve endings, supporting the theory that these channels mediate some forms of nociception [Cook S.P., Vulchanova L., Hargreaves K. M., Elde R. and McCleskey E. W. (1997) Distinct ATP receptors on pain-sensing and stretch-sensing neurons. Nature 387, 505-508]. The kinetics and pharmacology of ATP-gated currents in nociceptors suggest that the channels are comprised of either homomeric or heteromeric combinations of P2X3 receptors. Consistent with the diverse nature of P2X structure, electrophysiological responses of rat tooth-pulp nociceptors fall into two distinct classes based on desensitization and recovery kinetics. Here, we quantified the dramatic differences in desensitization kinetics of transient and persistent currents. The major component of transient P2X current desensitized with a tau decay = 32 +/- 2 msec, while persistent current desensitized > 100-fold more slowly, tau decay = 4000 +/- 320 msec. Both currents recovered from desensitization in minutes: tau recovery = 4 min for transient current, and tau recovery = 0.7 +/- 0.2 min for persistent current. Persistent current recovery was often accompanied by a current "overrecovery" that averaged ca threefold magnitude prior to desensitization. Comparison of ATP current in elevated Ca2+ext also revealed differences in transient and presistent currents. In 2 mM Ca2+ext medium, decrease of Na+ext resulted in an almost complete reduction of persistent, but not transient, current. Subsequent elevation of Ca2+ext greatly increased the transient, but not persistent, current. Mechanistic explanations for either the increase in transient current magnitude by elevated Ca2+ext, or persistent current overrecovery may reflect endogenous pathways for P2X receptor modulation.