<p>Psychophysical experiments were conducted to explore the mechanisms of interpolation and grouping in vertical and diagonal (45°) stimulus orientations. Estimates of the orientations of interpolated (mentally drawn through dots) and real lines were compared. Images were also presented in the presence of distractors – multiple strokes with the same slope, which, when superimposed on straight lines, form the Zöllner illusion. The distances between dots and the lengths of lines in the reference stimuli were varied. Slope estimation thresholds in the vertical orientation for interpolated and real lines were found not to differ for all the stimuli used, while thresholds were higher in the diagonal orientation. A significant difference in the orientation estimation bias (illusion) was also observed for real and interpolated lines. These data may indicate that there are differences in the mechanisms of estimation of the slopes of lines and interpolation in vertical and diagonal orientations, as well as different numbers of neurons with receptive fields with these orientations.</p>