1. The movement of labeled potassium ions has been measured across the innervated membranes of single isolated electroplaques, obtained from the organ of Sachs of Electrophorus electricus, mounted in an apparatus which allowed a separate washing of the two membranes.

2. Equations have been derived for a 3 compartment system in series in which tracer from a large pool in one outer compartment is collected in the other outer compartment. The amount of unlabeled ion in the middle compartment may be calculated and also the fluxes across the two membranes.

3. The flux of potassium across the innervated membranes of resting cells in a steady state was between 700 to 1000 µµmoles/cm.2/sec. and was unaffected by d-tubocurarine.

4. Direct stimulation of electroplaques with external electrodes caused an increase in the efflux of potassium from the innervated membrane of 5 to 8 µµmoles/cm.2/impulse, which was unaffected by d-tubocurarine; no change occurred in the efflux across the non-innervated membrane.

5. It is concluded that the discharge of electroplaques is accompanied by a small outward movement of potassium ions across the innervated membrane of the same order of magnitude as that found on excitation of squid giant axons. The data show a basic similarity of potassium movements across these two entirely different types of conducting membranes and suggest that this phenomenon may be a general feature of bioelectric currents propagating an action potential.

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