The leading edge of the a-wave of the electroretinogram (ERG) was evaluated as a measure of human cone photoreceptor activity. The amplitude of the cone a-wave elicited by flashes of different energy was compared to the predictions of a class of models from in vitro studies of cone photoreceptors. These models successfully describe the leading edge of the a-wave. Thus, the human cone a-wave can be used to test hypotheses about normal and abnormal cone receptors. The ability of the human cone to adjust its sensitivity in the presence of steady adapting lights was assessed by recording cone a-waves to flashes on adapting fields up to 3.9 log td in intensity and by comparing these responses to quantitative models of adaptation. The first 10 ms of the cone's response is little affected by field intensities up to 2.9 log td. The 3.9 log td field reduced the response to weak flashes by about a factor of 2.5 (0.4 log unit). This relatively small reduction in sensitivity can be attributed to a combination of response compression, pigment bleaching, and an adaptation mechanism that changes the gain without changing the time course. We conclude that either the human cones show relatively little adaptation or that they have an adaptation mechanism that involves a time-course change. That is, as we are limited with the a-wave to the first 10 ms or so of the cone's response, we cannot rule out a gain mechanism linked to a time-course change.