Block of the muscarinic K'current (ikch) by Ba has been studied in guinea-pig atrial cells using the whole-eeli patch-clamp technique. The dose. res. ponse curve for the block of，Kch can be fitted assum, ng that a muscarinic K'channel is blocked when a single Baion binds to it fapparent dissociation con. stant. Kd=125p-M at O mV. Block was voltage and time dependent. The vohage dependence can be explained bv Babinding to a site within the pore of the channel. 36% across the width of the membrane electric field (from the outside). Raising tlle bathing Kconcentration redticed Baoblock of iKch which sug, gests that Bao'and K'compete for a common bind. ing site. When Ba2'was added during all exposure to ACh (mascarinic K'channeopen). block of ikch developed rapidlL but when Ba2' was added prior to an exposure to ACh (muscalinic Kchannel closed). 1ittle block 0r iK Mb was evident when ACh was first applied. This suggests that when the muscarinic K'channel is closed in the absence of ACh. Bazdoes not have access to the binding site within the pore of the channel. In conclusion. Bablock of iK ACb iS con. centration. voltage, time, K' and state dependent.