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1. Flash illumination of a suspension of frog rod outer segments or rhodopsin solution in contact with a platinum electrode produces a rapidly developing negative displacement of potential of the electrode (with respect to a reversible electrode).

2. The amplitude of the potential change varies inversely with the H⁺ buffering capacity of the medium. It is inferred that the response is due to an uptake of H⁺ by the rod outer segments or rhodopsin, with the platinum surface acting as a pH electrode.

3. Determination of the action spectrum shows that the response depends on the absorption of light by rhodopsin.

4. In frog rods one acid-binding group with a pK of about 7·9 is produced for each molecule of rhodopsin bleached, consistent with a rhodopsin concentration in frog rods of 1·7 mM.

5. It is suggested that the time course of the response with rhodopsin solutions reflects the kinetics of the conversion of metarhodopsin I to metarhodopsin II.

6. A slower time course of voltage change observed for suspensions of outer segments is attributable to the time required for the diffusion of H⁺ buffer out of the rods.