The Kv currents are divided into three different K+ currents, such as slow inactivating transient K+ current (I(D)), fast inactivating transient K+ current (I(A)) and dominant sustained K+ current (I(K)), in small-diameter rat trigeminal ganglion (TG) neurons. The concentration of alpha-DTX (an I(D) blocker) to evoke the maximal inhibition of I(A) was 0.1 microM, and this concentration caused a 20 % inhibition of I(A) and increased the number of action potentials. Irrespective of the presence of 0.1 microM alpha-DTX, the application of 0.5 mM 4-AP (an IA blocker) caused a 51 % inhibition of I(A) and increased the number of action potentials. The responses were associated with the decreases in the resting membrane potential (RMP) and duration of depolarization phase of action potential (DDP). The application of 2 mM tetraethylammonium (TEA, an I(K) blocker) produced a 55 % inhibition of I(K). Irrespective of the presence of both I(D) and I(A) blockers, the I(K) was the predominant K+ current. The prolongation of duration of action potential was usually observed following TEA treatment, suggesting that I(A) and I(K) had independent effects regulating the intrinsic firing properties of the action potential number and timing, respectively. Furthermore, the response characteristics of action potentials in the presence of both 4-AP and TEA resemble those of TG neurons in rats following chronic constriction nerve injury of the infraorbital nerve as well as after inferior alveolar nerve section. Thus, reducing effects of both I(A) and I(K) may be useful to investigate the mechanism of allodynia.