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1. Membrane current was derived from simultaneous measurements of interstitial `dark' voltages and conductivities along the radial axis of frog photoreceptor cells. Membrane current was subsequently resolved into its component ionic currents or fluxes by means of ionic substitution and by the use of inhibitors of active transport.

2. The plasma membrane of the frog rod outer segment was found to be permeable to Na⁺ and Cl^-, with a ratio of Na⁺ to K⁺ permeabilities higher than that found in most neuronal cells. A net inward flux of 1·5 x 10⁸ Na⁺/sec.rod flows across the outer segment plasma membrane in the dark.

3. The proximal portion of the rod receptor, extending from the proximal region of the inner segment to the synaptic terminal, is mainly permeable to K⁺, although some degree of Na⁺ permeability is also presumed.

4. A hyperpolarizing electrogenic Na pump was localized to the base of the outer segment and inner segment of the cell. The pump transfers at least 10⁸ charges/sec out of the cell at this level, the pump current dividing and re-entering the cell at both the outer segment and proximal portion of the photoreceptor including the synaptic terminal.

5. These findings have been incorporated into an ionic model of the photoreceptor, and its implications for cellular functioning considered.