The a-wave of the photopic electroretinogram (ERG) of macaque monkeys is made up of the electrical activities of cone photoreceptors and post-photoreceptoral neurons. However, it is not known whether the contributions of these two components change in retinas with inherited photoreceptor degeneration. The purpose of this study was to determine the contributions of cones and post-photoreceptoral neurons to the a-wave of the photopic ERGs in rhodopsin Pro347Leu transgenic (Tg) rabbits.