Effects of internal spermine on outward single-channel currents through a strongly inwardly rectifying K(+) channel (Kir2.1) were studied at asymmetrical K(+) concentrations (30 mm external and 150 mm internal K(+)). The current-voltage (I-V) relation for the single channel was almost linear and reversed at -37 ± 3 mV (V(R); n = 19). The channel conductance was 26.3 ± 1.3 pS (n = 24). The open-time and closed-time histograms were fitted with a single exponential function. Internal spermine at a concentration of 1-100 nm reduced the open time of the outward currents in a concentration-dependent manner and produced a blocked state. The steady-state open probability of the outward current decreased with larger depolarizations in both the absence and presence of internal spermine. The steady-state open probability with asymmetrical K(+) and symmetrical (150 mm external and internal K(+)) concentrations plotted against driving force (V - V(R)) coincided with smaller depolarizations in the absence of spermine and larger depolarizations and higher spermine concentrations in the presence of spermine. The blocking rate constants and unblock rates with 30 mm and 150 mm external K(+) were similar at the same driving force. The dissociation constant-membrane potential relation for 30 mm external K(+) was shifted in the negative direction from that for 150 mm external K(+) by 36 mV. These results suggested that the blocking kinetics depends on driving force to produce driving force-dependent inward rectification when the equilibrium potential for K(+) is altered by changing external K(+) and that the energy barriers and wells for blocking ions from passing or lodging are not stable but affected by external K(+) ions.