The hyperosmolarity-induced response of the ocular standing potential (SP) provides a method of testing the function of the retinal pigment epithelium (RPE) without using light stimulation. In this study, the changes in potential level occurring after a short-term intravenous injection of 10 ml of 20% mannitol were determined by means of a direct current amplifier for the following groups: Group 1, normal rabbit eyes; Group 2, rabbit eyes in which the RPE was damaged by sodium iodate; Group 3, rabbit eyes in which the photoreceptors were damaged by monoiodoacetic acid; Group 4, rabbit eyes with uveoretinitis experimentally induced by Arthus-type inflammation. The following results were obtained: 1. The hyperosmolarity-induced SP response consisted of a transient increase in potential level (positive wave) in Group 1. 2. For Group 2 a transient decrease in potential level (negative wave) was obtained, i.e., a reversal of the normal positive response to negative wave. 3. A positive wave and no reversal was found for Group 3. 4. For Group 4 a negative wave and a reversal of the normal response was obtained. These hyperosmolarity-induced SP responses provide additional information concerning the possibilities of the method for studying the function of the RPE.