Contrast threshold and perceived orientation of a line segment were measured when another line segment was simultaneously presented either to the same or the other eye; the angle between the two line segments was varied. The presence of the masking line elevated the contrast threshold under both conditions and the threshold increased similarly both in monoptic and dichoptic masking when the masking angle was made smaller. The presence of the masking line affected also the perceived orientation of the test line. The effect was similar both in monoptic and dichoptic masking; the largest change of perceived orientation occurred at about 15 degrees masking angle and the change was smaller at other angles. The effect disappeared within a short distance when the masking line was removed farther from the test line. The similarity of the monoptic and dichoptic threshold elevations demonstrates that there are lateral inhibitory interactions between central neural units in the human visual system. It is likely that the interacting units mediate the perception of contour orientation, for the threshold elevation functions were consistent with concurrent changes of perceived orientation. The results are evidence for the hypothesis that inhibition between orientation detectors is a factor in the perceptual expansion of acute angles.