1. It was shown that the potentials evoked by using a grating pattern alternated in phase at 8 Hz is proportional to the logarithm of the supra-threshold contrast. Other functions were considered, but they did not describe the data so parsimoniously.2. The same logarithmic function described the results when a grating was simply flashed on and off; therefore, the apparent movement accompanying the phase alternations is not necessary to evoke the potential.3. The contrast at which the evoked potential reached the theoretically zero voltage (C(0)) was compared with the psychophysical contrast threshold, determined by means of proportion-of-time seen measurements; the C(0) contrast corresponded to 50% time seen.4. The potential, corrected for the proportion-of-time seen, was found linearly related to contrast.