Large conductance voltage and Ca2+-activated K+ channels (BK channels) ， which are ubiquitously expressed among mammalian tissues, play a variety of physiological roles. The inactivating BK channels have not been extensively studied in primary tissues and rarely do they exhibit inactivation. An inactivating BK current (BKi) was identified in rat small dorsal root ganglia (DRG) neurons . The inactivating currents are sensitive to the specific BK channel blocker charybdotoxin (ChTX) and the effect of ChTX can be removed partially when washed with normal external solution. The inactivation of BKi can be removed by external treatment of papain.