Orientation discrimination threshold is a monotonically increasing function of retinal eccentricity. Increasing stimulus length extends the range of eccentricities over which fine orientation discriminations can be made. Orientation discrimination thresholds at all eccentricities are determined by the size of the cortical image of the stimulus. Thresholds obtained using either nasal or temporal hemiretina are similar up to the blind spot, beyond which the temporal retina yields increasingly higher thresholds. The results are consistent with a recent theoretical study which predicts that orientation discrimination threshold is determined by the number of cortical cells activated by the discrimination target.