Visual thresholds measured at the central and midperipheral visual field have been tested in the eyes of a series of 10 healthy normal persons using a monochromatic red test light object (Goldmann area III, 25' of arc in the visual field) of 632 nm wavelength presented on white backgrounds of 150 cd/m2 (5000 K) and 1 cd/m2 (2800 K) luminance, as well as with no background illumination following a 45 min period of dark-adaptation. Another series of 10 normal subjects were tested with an equally large white (2800 K) test object shown on a white background of 1 cd/m2 (2800 K) luminance. Results obtained in congenital and acquired achromatopsia as well as in three relatively mild cases of rod-cone dystrophy have been related to the normal thresholds.