The patch-clamp technique of cell-attached and inside-out configurations was used to study the single potassium channels in isolated guinea pig hepatocytes. The single potassium channels in isolated guinea pig hepatocytes were recorded at different K + concentrations. A linear single-channel current-voltage relationship was obtained at the voltage range of -80 to -20 mV with slope conductance of 70 * 6 pS (n = 10). Under symmetrical high Kt concentration of 148 m M in the cell-attached patch membrane, the I-V curve exhibited a mild inward rectification at potentials positive to +20 rnV. The values of reversal potential was +5 2 2 mV (n = 10). When the external potassium concentration ([K+],) was decreased to 74 m M and 20 mM, the slope conductance was decreased to 48 & 2 pS (n = 4) and 24 * 3 pS (n = 31, respectively. The reversal potential was changed by 58 mV for a tenfold change in [Kfl,, indicating that this channel was highly selective for K+. Open probabilities (Po) of the channel were 7 3 4 3 % without apparent voltage dependence. The distributions of open time of the channels were fitted to two exponentials, while those of closed time were fitted to three exponentials, exhibiting no voltage dependence. The success rate of K+ channel activity to be recorded was 28% at room temperature, and there were no increases in the success rate nor in the channel opening probabilities at a temperature of 34-36ยฐC. P , in inside-out patches was not changed by application of 1 pM Ca2-' nor 1 m M Mg2+ to the internal side of patch membranes. It is concluded that a novel type of the K+ channels in guinea pig hepatocytes had different properties of slope conductance, channel kinetics, and sensitivity to [Ca2+],, from those in other species. o 1994 Wiley-Liss, Inc