The neural correlates of stereopsis are unknown. A large volume of data has now been collected upon the responses of striate cortex units to binocular stimulation, and it has been implied that such binocular units serve as the neural basis for stereopsis. Most of these data have been collected on cat. The recent demonstration that an operant response of cat can be brought under the control of binocular stimuli suggests several questions which this program attempts to answer. (1) What is the visual acuity of the common and Siamese cat? (2) Can behavior based on binocular disparity cues be demonstrated repeatedly and reliably in the common and Siamese cat? (3) Can behavior based on stereopsis be demonstrated repeatedly and reliably in the common and Siamese cat? (4) How do binocular striate cortex units in cat code for stereoscopic visual stimulation? (5) Do binocular striate cortex units demonstrate a systematic change in activity in close temporal association with a behavioral response under the control of stereoscopic stimuli?