Ion-sensitive microelectrodes were used to measure intracellular activities (aix) of Na+, K+, and C-1 in Balanus photoreceptors. Average values of aiNa, aiK, and aiCl were 28 mM, 120 mM, and 65 mM, respectively. Equilibrium potentials calculated from these average values were: Na+ +64 mV, K+ - 77 mV, and and Cl- -42 mV; ther average value of the resting potential for all cells examined was -41 mV. Long exposure to intense illumination produced measurable increases in aiNa. Classical Na+ - K+ reciprocal dilution experiments were analyzed with and without observed changes in aiK. As aoK was increased, the membrane depolarized, and aiK increased. Better agreement was found between the membrane potential and the directly determined EK than expected from the standard relation between Em and aoK. The latter produced pNa:pK estimates of the resting photoreceptor membrane that were higher than estimates based on data from the ion electrodes. Generally, Em was more negative than EK as aoK was increased. This is consistent with a significant chloride permeability in the dark-adapted photoreceptor.