1. The activity of neurones of the visual cortex (area 17) has been recorded in anaesthetized cats in response to gratings of different profile and to single light and dark bars. 2. At very low spatial frequencies, outside the frequency response range to sinusoidal gratings, the response to square-wave drifting gratings is obtainable from a combination of the response to the single bars of the grating presented in isolation. At higher spatial frequencies this is no longer true. 3. At very low spatial frequencies the responses to square-wave gratings and to missing-fundamental gratings (obtained by subtraction from the square-wave grating of its fundamental gratings (obtained by subtraction from the square-wave grating of its fundamental harmonic) are very similar. 4. At spatial frequencies near the peak of the spatial frequency tuning curve of the cell, the responses to square-wave grating and to sinusoidal gratings are very similar. At these spatial frequencies the response to the missing-fundamental grating is practically zero. 5. At spatial frequencies lower than that of best sensitivity for the cell, the response to square-wave gratings is correlated with the 1st and 3rd harmonic of the stimulus. 6. We conclude that at very low spatial frequencies of the grating the response of cortical cells is correlated with the light or dark edges (or light or dark bars) of the stimulus, because the edges contain high frequencies within the range of sensitivity of the cells. At higher spatial frequencies the results are interpreted best by assuming that cortical cells respond to the harmonics of the visual periodic stimulus. 7. When a background of dynamic visual noise is added to increase the spontaneous discharge of simple cells, their response to visual stimuli becomes linear or quasi-linear. The stimuli could be either single bars or gratings.