When frog skeletal muscle fibres are bathed in solutions containing Cs(+) and K(+) in the ratio 1:4,000, a reduction is observed in the size of inward K currents through the resting membrane. This effect is enhanced by an increase in either hyperpolarisation or external Cs(+) concentration. It can be predicted from these findings that regenerative changes in membrane potential should be obtainable in fibres, in the presence of Cs(+), that are hyperpolarised by means of a current electrode. Such responses are described in the last part of this report. In squid axon and frog node, internal Cs(+) produces a voltage-dependent block of the delayed, outward K currents, though the ratio of Cs(+) to K(+) required for this effect is far greater than that used in the experiments reported here. A closer parallel can be drawn between our findings and those recently reported on the inward K currents in the starfish egg cell.