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1. The mechanism of reduction and final abolition of the depolarizing receptor potential of Balanus eburneus photoreceptors in K+-free saline was examined with electro-physiological techniques including voltage-clamp and ion specific electrodes. 2. An extended exposure to K+-free saline reduces the transient peak and the steady phases of the depolarizing receptor potential by approximately equal amounts. The process can be reversed in normal saline although the wave form of the response is often more rectangular upon recovery. Restoration of K+ induces a transient hyperpolarization of the resting membrane for several minutes. 3. The depolarizing receptor potential can also be restored in K+-free solution by reducing the Ca2+ concentration. This saline depolarizes the resting membrane, and the wave form of the depolarizing receptor potential assumes a rectangular configuration. 4. Voltage-clamp experiments revealed that an extended exposure to K+-free saline produced an extreme reduction of the inward light-induced current (LIC), but no detectable change in the membrane potential at which the current reverses sign. Membrane conductance in darkness showed little change. Reduction of the Ca2+ concentration from 20 to 0-2 mM in K+-free restored the current and produced a negative 8-10 mV shift in the zero current potential. There was also a significant decrease in membrane conductance in darkness. 5. Current-voltage relations of the membrane in K+-free, low Ca2+, or K+-free low Ca2+ salines were somewhat dependent upon the order the salines were presented. 6. Low Ca2+ saline (0-2 mM) by itself produced a -5 mV shift in the zero-current potential. Removing K+ in low Ca2+ produced an additional shift (-5 mV) in the zero-current potential.