The light-induced membrane voltage response (receptor potential, ReP) and the absorption change of the intracellularly injected calcium indicator arsenazo III (arsenazo response) were recorded simultaneously in Limulus ventral nerve photoreceptor cells. A double pulse technique was applied for stimulation. After pressure injection of the indicator into the cell absorption changes were measured at 646 nm to obtain a measure of the changes of the intracellular calcium ion concentration. 1. The size of the arsenazo response increases with increasing intensity of the light stimulus. The intensity dependence of the size of the arsenazo response delta A max shows almost no correlate with the peak amplitude of the ReP, but correlates rather well with the time integral of the ReP. 2. Decreasing light adaptation (caused by prolongation of the repetition time of the pulse pairs) leads to an increase in size of the arsenazo response. Also here delta A max correlates better with the time integral of the ReP than with its peak amplitude. 3. Lowering the calcium concentration in the superfusate (from 10 mmol/l to ca. 40 mumol/l) causes after ca. 10 min a drastical diminution of the arsenazo response to values below the noise level, and a less marked reduction in size of the ReP. The falling phase of the ReP is slower. After return to normal calcium concentration the arsenazo response recovers partly in ca. 50 min, while the ReP recovers faster. The results show two opposite correlation between ReP and arsenazo response: Increase in size and duration of the ReP causes a greater transient increase of the intracellular calcium ion concentration. This in turn tends to reduce and shorten the ReP. Which effect dominates obviously depends on the conditions of the experiment: when the calcium concentration in the superfusate is reduced to ca. 40 mumol/l a slow decrease of the ReP is accompanied by a small increase of the intracellular calcium ion concentration.

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