1. The evoked potential in response to a grating alternating in phase at 8 c/s was recorded as a function of contrast from the occiput of man.2. It was found that a linear relation exists between the log. of contrast and the amplitude of the evoked potential.3. Extrapolation to zero amplitude voltage of the regression line between the amplitude of the evoked potential and log. contrast predicts the psychophysical threshold. This law was found to hold over the wide range of spatial frequencies tested.4. Below 3 c/deg the results are best fitted with two regression lines; one of these is generated from the foveal and the other from the parafoveal representation in the cortex.5. The slope of the regression lines was found to be almost independent of either the spatial frequency or the area of the stimulus grating.6. The slope of the regression lines could be markedly increased by using as a stimulus either two different spatial frequencies, or two different orientations, presented simultaneously.7. Using the evoked potential the selectivity to orientation was found to be so high that a channel was not influenced by another orientation 15 degrees away.8. The channels selectively sensitive to spatial frequency were highly selective and were not influenced by another spatial frequency one octave removed in spatial frequency.9. It is concluded that in man there exist neurones highly selective to both orientation and spatial frequency.