Several writers have hypothesized that the Mueller-Lyer illusion is influenced by brightness contrast between figure and background. According to them, lateral inhibition in the retina causes displacement in Mueller-Lyer figures at the intersections of lines. They argue that the strength of the displacement varies with brightness contrast between the background and the figure, and they cite research by Pollack and others for support. The experiments reported here show that the Mueller-Lyer illusion is no more affected by changes in contrast than are judgment of single straight lines. These results are not consistent with the view that lateral inhibition in the retina has an effect on the illusion.