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1. An isolated frog retina was mounted in an impedance chamber and superfused on its vitread surface.

2. Changes in the real part of the impedance (R) and also in the imaginary part were measured using alternating current in the frequency range 1-300 kHz passed from one surface of the retina to the other.

3. Under most conditions, the response to a flash of light, measured at frequencies below about 100 kHz, was a decrease in the real part of the impedance (R < 0).

4. The geometry of the electrodes was such that the system was particularly sensitive to changes in the impedance of the layer of photo-receptor outer segments. It was confirmed that most of R did arise here and that it was mediated by the absorption of light in rod photo-pigment.

5. The magnitude of R increased when the channels between the outer segments were constricted, e.g. by osmotic swelling of the outer segments. In addition to this increase, a further increase was seen following the commencement of recording in most of the experiments from which usable measurements were obtained.

6. In such retinas, the magnitude of R was greatest when measured at a frequency in the range 3-32 kHz, the largest changes being of the order of 0·1% of the resting value. A light flash bleaching about 1% of the pigment was sufficient to produce this. The onset of R was apparent within 1 msec of the flash (at 15° C); it reached a maximum in 0·5-4 sec and then returned towards the base line.

7. The main component of R was attributed to a decrease in the resistance of the spaces between the outer segments. In addition, there was a resistance increase which occurred at some other site, probably the surface membrane. The first of these components had an onset slightly more rapid than the second, and both occurred irrespective of whether the major cation in the superfusate was sodium or potassium.