The possibility that visual thresholds fluctuate in a circadian rhythmic (twenty-four hour) pattern was examined. Subjects were tested at 0400h, 1000h, 1600h and 2200h in a 30 minute dark adaptation procedure using a von Békésy tracking method. Two experiments were performed; the first tested cone functions with a red 1 degree monochromatic test stimulus located ten degrees nasally. There were no sex differences for the ten male and female subjects used in this study. However, there were time of day differences with higher thresholds at 1000h than at any other time. The second experiment examined both rod and cone thresholds with a heterochromatic test stimulus. Sex differences were found 'for the 10 male and 10 female subjects. The rods for males were found to exhibit significantly higher thresholds at 1000h than any other time. There were no significant differences for females. The null results for females may be due to an influence of the menstrual cycle on vision. The rod differences for males is thought to occur because of rod outer segment shedding which also has a circadian rhythm. A circadian rhythm questionnaire was administered to the subjects and it was found that morningness and vigorousness related to visual thresholds. / Master of Science