1. Patch clamp techniques were used to study the action of external Cs+ and Ba2+ on adenosine 5'-triphosphate (ATP)-dependent K+ channels in sarcolemmal vesicles from frog skeletal muscle. Both ions block channels in a voltage-dependent fashion, block increasing with hyperpolarization. 2. The Cs+ block is flickery, mean unitary current being reduced and open-level noise increased. The concentration dependence is consistent with 1:1 binding, with a dissociation constant (Kd) of 4.1 mM at -62 mV. The Kd increases e-fold for a 20 mV depolarization. 3. The kinetics of Cs+ block were analysed by amplitude distribution analysis, and by measurement of the excess open-level variance. Both methods gave similar rate constants for blocking and unblocking; about 20 mM-1 ms-1 and 75 ms-1 at -62 mV. 4. All the voltage dependence of the Cs+ block appears to lie in the blocking reaction; unblocking is independent of voltage. 5. Ba2+ blocks with slower kinetics, so that blocking events can be resolved in single-channel records. Ba2+ reduces mean open time and causes long closings. 6. The blocking rate constant for Ba2+ was measured from the open times. It was about 1.7 mM-1 ms-1 at -62 mV and increased e-fold for a 40 mV hyperpolarization. The unblocking rate, measured from closed times, yielded a Kd of about 0.1 mM at -62 mV, in agreement with that measured from the reduction in open-state probability. 7. Our results suggest that Cs+ and Ba2+ block at sites within the channel, and provide evidence that the channel is a multi-ion pore.