During intracellular iontophoretic injection of Ca++ into Limulus ventral photoreceptor cells, there is a progressive diminution of the light response. Following Ca++ injection, the size of the light response slowly recovers. Similarly, there is a progressive diminution of the light response during intracellular injection of Na+ and recovery after the injection is stopped. The rate of diminution during Na+ injection is greater for higher [Ca++]out. In solutions which contain 0.1 mM Ca++, there is nearly no progressive decrease in the size of the light response during Na+ injection. Intracellular injections of Li+ or K+ do not progressively decrease the size of the light response. We propose that an increase in [Na+]in leads to an increase in [Ca++]in and that an increase in [Ca++]in by any means leads to a reduction in responsiveness to light.

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