The threshold values for stereopsis with alternating monocular presentation of the corresponding member of a stereo pair was examined in relationship to the stimulus duration and the interocular delay. The maximum interocular delay for short stimulus durations was approximately 190 msec. The maximum stimulus duration was between 400-500 msec. At this stimulus duration the interocular delay must approach zero if stereopsis is to be maintained. In the threshold region the stereo effect diminished. One can possibly explain this phenomenon as a partial sensorial fusion. For normal appreciation of stereopsis, the critical frequency was about 2 Hz over the threshold values found with a given stimulus duration.