To investigate binocular single vision, we examined monocular contrast sensitivity during binocular fixation by changing the intervals between the beginning of fixation and a probe stimulus, within 10 seconds. Monocular contrast sensitivities were quite stable within 1s of the interval delay in both eyes, but they were reduced in either eye if the interval delay was more than 1s (Experiment 1). In Experiment 2, a similar stimulus was monocularly presented. In this case, decline of contrast sensitivity was not observed in either eye. In Experiment 3, when the stimulus was interrupted briefly before the probe presentation, the contrast sensitivity was recovered. These results suggest that after prolonged viewing the binocular system does not sustain either eye sensitivity equally unless there is interruption of the binocular stimulation.