In the present study, ATP-activated currents (I(ATP)s) recorded from rat trigeminal ganglion (TG) neurons using whole-cell patch clamp technique are classified into three types (F, I and S) based on the characteristics of their activation and desensitization. The time of rising phase (R(10-90)) of types F, I and S of I(ATP) is measured to be 33.6+/-4.5, 62.2+/-9.9 and 302.1+/-62.0 ms respectively, and positively correlated to cell size. The time of decaying phases (D(10-90)) of types F and S is 399.4+/-58.2 and >1500 ms, respectively. The dose-response curves for the three types of I(ATP) show that their EC(50) values are close (3.44 x 10(-5), 4.89 x 10(-5) and 4.14 x 10(-5) M for types F, I and S respectively, P>0.05). Their reversal potentials are basically the same, varying from +4 to +10 mV. In addition, using whole-cell patch clamp technique in combination with single cell immunohistochemical staining for P2X receptor subunits, our results suggest that the type distinction of ATP-activated current was associated with cell size and P2X receptor subunits: small-sized cells with type F of I(ATP) express only P2X1 and/or P2X3 subunits, while cells with types S and I of I(ATP) express P2X2 or P2X4 in addition to P2X1 and P2X3.