The component P III of the electroretinogram was recorded after stimulation with short stimuli with different luminance. It is shown that the impulse response function of a linear second order model can be well fitted to the recordings. Two of its parameters, after logarithmic transformations, are linearly dependent on the luminance while the third parameter varies only within a small interval. It is therefore possible to describe the relation between P 111 and luminance in a linearized second order model. Physiological implications of these results are discussed.