We measured the variability of the b-wave of the electroretinogram as a function of stimulus luminance in two young normal individuals. We also estimated b-wave variability by examining residuals from Naka-Rushton curves fit to intensity-response data. The change of variability with amplitude was similar with both techniques. The standard deviation of b-wave amplitude rose with b-wave amplitude at low stimulus intensities. At higher intensities, the standard deviation of b-wave amplitude became constant. The point at which the standard deviation became constant was Log K for the eye, as determined by fitting the data with the Naka-Rushton equation. These changes suggest that the mechanisms underlying the growth of the b-wave with luminance change near Log K.