SOMATO-DENDRITIC subthreshold transient potassium current (ISA) was measured in acutely isolated rat hippocampal CA1 pyramidal neurons. The inactivation of this current was insensitive to externally applied H2O2(20 mM) which causes cysteine oxidation. This result suggests that Shal-channels not Shaker Kv1.4 channels underlie the somato-dendritic ISAin rat CA1 pyramidal neurons. The kinetics of the ISAinactivation was measured at various [K+]out. Increase in [K+]outleads to acceleration of Shal-channel inactivation. Thus, the shift in [K+]outfrom 1 to 50 mM results in decreased inactivation time constant from 37 to 91ms. This effect of [K+]outon the ISAis opposite to the previously described action of [K+]outon the inactivation of Shaker K+channels.