Background: The tendency for the electrophysiologic effect of class III antiarrhythmic agents (action potential prolongation) to be diminished at faster heart rates represents a major drawback of this class of drug and is usually referred to as "reverse use dependence." A novel class III agent, MS-551, has recently been reported to exhibit less reverse use dependence than E-4031. We set out to investigate whether this observation may be due to differential blockade of the inward rectifier current (iK1) by these drugs. Methods and Results: We recorded iK1 using single channel methods and cell attached patch configurations, with standard patch clamp technology. Neither E-4031 nor racemic sotalol in concentrations up to 100 μM had any significant effect on the open probability or kinetics of iK1 channels. MS-551 produced a concentration-dependent reduction in the open probability of i K1 without altering the single-channel conductance. Openings to subconductance levels were abolished in three of six patches in which they had been frequently present in the absence of drug. MS-551 had no effect on mean channel open time but increased the slower component of the closed time. Conclusions: MS-551, unlike E-4031 and sotalol, appears to produce significant blockade of the inwardly rectifying potassium channel at clinically relevant concentrations. We propose that this might provide a partial explanation for the observed differences in their response to rate changes.