Photoreceptor somas in the ventral rudimentary eye of Limulus polyphemus were impaled with three micropipettes. Two micropipettes were connected in a voltage-clamp circuit and the cells were stimulated by brief flashes. The third micropipette did not measure any significant deviations from the 'clamped' voltage during responses to the flashes, in several geometries of electrode placement, even for very bright flashes. Therefore using the described techniques there is no evidence for spatial non-uniformity of intracellular voltage in the soma of these photoreceptors. 2. A voltage-sensitive dye was used to monitor light-induced changes in membrane voltage while intracellular voltage was held clamped by a feed-back circuit. With a known series resistance connected between the bath and ground the dye recorded a light-induced change in membrane voltage. When there was no added series resistance, the light-induced change was smaller and often undetectable. From these data the naturally occurring series resistance was calculated to be less than or equal to 30 k omega. 3. From these measurements, as well as from calculations for a model spherical cell, we conclude that membrane potential can be controlled to within 2 mV using our micropipette 'point clamp' methods, for all but the brightest stimuli.
