K⁢ currents play multiple roles in the excitability of dorsal root ganglion (DRG) neurons. Influences on these currents change the shape of the action potential, its firing threshold and the resting membrane potential. In this study, whole cell configuration of patch clamp technique had been applied to record the blocking effect of capsaicin, a lipophilic alkaloid, on the delayed rectifier K⁢ current in cultured small diameter DRG neurons of adult rat. Capsaicin reduced the amplitude of K⁢ current in dose dependent manner, and the concentration-dependence curve was well described by the Hill equation with KD value of 19.1μM. The blocking effect of capsaicin was reversible. Capsaicin (10 μM) shifted the steady- state inactivation curve in the hyperpolarizing direction by about 15 mV and increased the rate of inactivation. The voltage dependence of activation was not affected by capsaicin. These multiple effects of capsaicin may suggest that capsaicin bind to the region of K⁢ channel, participating in inactivation process.