The major goal of this investigation was to describe and characterize response components of graded retinal responses in man and cat as a function of check size of the spatial stimulus. The arterially perfused cat eye enabled us to record the pattern reversal electroretinogram (PERG) and simultaneously summed activity of retinal ganglion cells by recording a compound action potential of the optic nerve (ONR). Checkerboard pattern reversal stimuli were presented at a low temporal frequency (recording 2 reversals in each trace), while a number of parameters such as check size, modulation depth and field size have been varied in order to elucidate similarities in response characteristics between the PERG and ONR. The striking similarity area dependence of the PERG and ONR points to ganglion cell activity as well as summation within the latter's receptive field center, reflected in the PERG. A fractionation was obtained of the PERG into two separate components generated by specific changes in spatial contrast and by luminance, respectively. The spatial response component of the PERG recorded in man from the enucleated cat eye closely resembled and consisted of a negative deflection. This component is preceded by a positive deflection sensitive to luminance changes and prominent upon peripheral retinal stimulation. It is anticipated that mild to moderate abnormalities of the ganglion cell activity will be first reflected in the negative contrast component upon central retinal stimulation.