Psychophysical luminance thresholds were taken from dark-adaptation curves representing two-hour intervals around the clock to identify diurnal changes in photopic and scotopic luminance sensitivity. Dark adaptation curves were obtained by tracking thresholds for a 2 deg achromatic stimulus located 10 deg temporal to the fovea of the left eye following a one min bleach. Test sessions were randomly distributed and each of the three subjects was tested twice at each time over a two-week period. The results showed small changes in both photopic and scotopic threshold over time of day. The amplitude of photopic threshold changes was small (0.1 log unit) and the time of peak sensitivity varied among individuals. Scotopic thresholds were highest at about 0230 hr, and the amplitude of threshold changes was larger (0.12-0.24 log unit). Oral temperatures measured at the time of threshold testing showed a moderate inverse correlation with thresholds; when temperature was low, thresholds were high. The threshold changes were not related to known rhythms in retinal physiology, but may be related to perceptual variables.