The hyperosmolarity response of the standing potential was recorded in retinitis pigmentosa (20 eyes), central (pericentral) retinitis pigmentosa (4 eyes), pigmented paravenous retinochoroidal atrophy (2 eyes), fundus albipunctatus (8 eyes), and Stargardt's disease (or fundus flavimaculatus) (14 eyes). The light peak/dark trough ratio (the L/D ratio) and the Diamox response were also determined. The hyperosmolarity response was greatly suppressed (less than M-4SD; M and SD indicate respectively the mean and the standard deviation in normal control subjects) in all examined eyes with retinitis pigmentosa (20 eyes) including retinitis pigmentosa sine pigmento (8 eyes), central (pericentral) retinitis pigmentosa (4 eyes), and pigmented paravenous retinochoroidal atrophy (2 eyes). The L/D ratio was larger than 1.26 (M-2.5 SD) in the half of the eyes with the above-described diseases. The hyperosmolarity response was abnormal (less than M-2 SD) in 4 of 8 eyes with fundus albipunctatus. The L/D ratio was normal in all 8 eyes. The hyperosmolarity response was abnormal (less than M-2 SD) in all 14 eyes with Stargardt's disease or fundus flavimaculatus. The L/D ratio was abnormal in 5 of these 14 eyes. The hyperosmolarity response was more frequently abnormal than the L/D ratio in the chorioretinal dystrophies mentioned above, and hence is useful particularly for early diagnosis of these disorders.