In the chicken ERG, recorded in response to white light stimulation, the amplitudes of the a-wave and the b-wave are linearly related, irrespective of the level of light adaptation. This linear relationship holds true for any monochromatic stimulation, but the straight lines representative of the a-b relationship are more or less shifted towards higher b-wave amplitudes, depending on the wavelength. Thus, the amplification between a-wave and b-wave appears to be wavelength-dependent in the chicken ERG. On the contrary, in the rat, the a-b relationship is more compressive, and it does not change with stimulus wavelength. However, it is markedly modified by light adaptation. The extent to which these differences in the initial stages of brightness coding and light adaptation result from differential features in the functional organization of photopic and scotopic retinae is discussed.