The threshold for detecting a deviation in the orientation of a test stimulus from vertical and from horizontal was measured with a single test interval. The test stimuli were step edges, thin lines, and sine wave gratings of 2.5, 5.0 and 10.0 c/deg. Recognition thresholds were found to be independent of the frequency content and nature of the test stimulus. The average threshold value on the principal meridians was found to be approx. 0.6 deg. Additional testing was conducted with two interval forced choice designs, both with and without random perturbation of the orientation of the stimuli. Random perturbation of orientation reduces acuity estimates. The results are compared with those obtained by testing on the oblique meridia. We conclude that the meridional anisotropy of orientation discrimination, which favours the vertical and horizontal is a result, in part, of the influence of non-visual mechanisms.