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The authors investigated and specified the spatial extent of binocular suppression for normal and strabismic subjects at differing retinal eccentricities. Increment luminance thresholds for detecting a probe line which was presented to one eye were measured under the influence of a contralateral suppressing line. Probe sensitivities were obtained as a function of positional disparity of the contralateral suppressing line. Two normals and two strabismics served as subjects. The suppression patterns for the normal and strabismic subjects were similar except for the width of the suppression area. The interocular suppression functions were U-shaped, and there were significant effects of eccentricity. The spatial extent of suppression for normal subjects was between 2 and 20 min arc at the fovea or between 5 and 40 min arc at an eccentricity of 2 degrees. The maximum disparity which produced suppression effects in the strabismics was about half of that obtained for the normals, in either the foveal or near peripheral conditions. The psychophysically obtained U-shaped functions appear to be similar to the neuro-physiological data from cat striate cortex by Kato, Bishop & Orban (1981). Therefore, the authors believe that these results may reflect the operation of an interocular version of the Westheimer function.