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1. Potentials were recorded from the cytoplasm and from the vacuole of leech photoreceptors. Since the vacuole is lined with microvilli and is connected to the outside by narrow channels, the potential drops between vacuole and outside measure the current through the microvillar membrane.

2. In darkness, the potential of the cytoplasm with respect to the outside is about — 45 mV while the potential of the vacuole is approximately zero.

3. Following illumination the negativity of the cytoplasm decreases and the vacuole becomes negative relative to the outside.

4. For dim intensities, the response to a flash of light may grow proportionately more than the intensity of the flash. This is probably due to development of a depolarizing local response.

5. The resistance from the cytoplasm to the outside was about 150 M in darkness and decreased to approximately 40 M at the peak of the response to a bright flash (on average). Corresponding measurements from the vacuole gave 50 M in darkness and 35 M at the peak of the response.

6. Charging curves produced by steps of constant currents applied to the cytoplasm or to the vacuole include two time constants (about 5 and 50 msec on average). The longer time constant decreases greatly with bright illumination.

7. The results are consistent with the interpretation that the response to light is brought about by an increase of conductance of the microvillar membrane.