The spatial response function of the electroretinogram (ERG) to contrast checkerboard pattern reversal at several check sizes was determined at a fixed contrast. The influence of the rate of modulation on the spatial response function was assessed. Reversing square wave patterns were presented at eight temporal frequencies ranging from 1 to 25 reversals per sec. The waveform consisted of an initial positive and a subsequent negative deflection. Irrespective of the temporal frequency, the spatial response function of the positive component did not show a spatial tuning. The amplitude of the negative component exhibited a pronounced attenuation of the response at check sizes larger than optimal. Mean maximal amplitude was found at an optimal check size between 25 and 50 min of arc. A distinction between a positive or negative component was not made for temporal frequencies higher than 10 reversals per sec, since the waveform at these modulation rates consisted merely of a sinusoidal steady-state response. The spatial response function obtained at 14 reversals per sec, resembling that of the negative component, exhibited a prominent spatial tuning. The results demonstrate that the pattern ERG has at least two components: a positive component which is not specific to changes in retinal distribution of contrast, followed by a negative wave showing spatial tuning across temporal frequency.