The corneally recorded rod photocurrent component (photoresponse) underlying the a-wave feature of the electroretinogram was analyzed. The results set empiric limits on critical photoresponse variables. Measurements were obtained from four normal adult subjects on a-wave amplitude, a-wave velocity, b-wave amplitude, b-wave implicit time and b-wave height above baseline. At high intensity, interference from the b-wave component was minimized and the amplitude of the saturated photoresponse component was approximated by the a-wave feature. At lower intensities, the a-wave feature represented progressively less of the underlying photoresponse amplitude. Photoresponse amplitude saturation was signaled by the abrupt slowing of the rate of decline of b-wave peak latency and occurred at an intensity about 2.5 log units above the first appearance of the b-wave. At the intensity of photoresponse saturation, the peak amplitude of the a-wave feature was only about 25% of the maximum amplitude of the underlying photoresponse component. A-wave leading edge velocity was found to increase up to 3 log units above the intensity of photoresponse amplitude saturation and to provide a good estimate of photoresponse velocity at higher intensities. A cascaded low-pass filter model with modifications to accommodate amplitude and timing nonlinearities was used to generate a set of probable underlying photoresponses from the analysis of a-wave amplitude and velocity. Movement of the a-wave leading edge to the left at higher intensities in algebraic combination with a static b-wave leading edge above the intensity of photoresponse amplitude saturation was found to explain the second rise of the b-wave amplitude function and the decline of b-wave amplitude above baseline at high intensities. This analysis provides a basis for modeling the underlying photoresponse on a biochemical level and for interpreting photoreceptor damage in disease states.