1. The excitatory and inhibitory components in the receptive fields of unimodal simple cells in the striate cortex of the cat anaesthetized with nitrous oxide have been described using slits of light and single light-dark edges as stimuli.2. There is a small excitatory region (excitatory complex) centrally located in the receptive field that is made up of various combinations and spatial arrangements of subliminal excitatory and discharge subregions or centres.3. The subliminal excitatory centres were revealed by a binocular facilitation technique. The excitability of the cell was raised by repeated stimulation via one eye while the neurone was tested with single edges via the other eye.4. The subliminal excitatory and discharge centres are each specifically activated by only one type of edge, light-dark or dark-light, and then only in one direction of motion. All the subregions in the excitatory complex have the same optimal stimulus orientation.5. Inhibitory components in the receptive field were identified by stimulating the cell with bars of light and single edges against an artificial background discharge produced by repeated stimulation separately applied either to the same eye (monocular conditioning) or to the other eye (binocular conditioning). There are powerful inhibitory sidebands to either side of the excitatory complex and these inhibitory regions merge to include the excitatory complex when stimulus orientation is angled away from the optimal.6. Excitation is highly stimulus specific whereas inhibition is non-specific.7. The organization of the two receptive fields of a binocularly discharged cell can be closely similar.8. The attempt is made to translate the concept of subliminal excitatory and discharge centres into specific neural mechanisms involving both the geniculo-cortical input and various intracortical circuits.9. These new developments call for only minor modifications to the model we have proposed for the organization of the receptive field.