1. Rotation of an object about its horizontal axis, towards or away from the viewer's eyes, usually causes the images of its contours to have slightly different orientations on the two retinae.2. We recorded action potentials from binocular neurones in the cat's visual cortex and measured their orientation-selectivity carefully in both eyes.3. The optimal orientation for a single cell is not necessarily identical on both retinae. For a large sample of cells there is a range of more than + 15 degrees (S.D. about 6-9 degrees ) in the difference of preferred orientation in the two eyes. These interocular differences in receptive field properties cannot be attributed to rotation of the eyes or to the errors of measurement.4. During simultaneous binocular stimulation the images must not only lie in the correct place on both retinae but also have exactly the right orientation for both receptive fields in order to elicit the maximum response from a neurone.5. Therefore certain binocular cells respond specifically to objects tilted in three-dimensional space towards the cat, or away from it.