PURPOSE: The effect of form deprivation myopia (FDM) on cone photoreceptor function was investigated. METHODS: Photopic electroretinogram (ERG) a-waves were recorded from control and form-deprived eyes of chickens. Cone-generated P3 responses were derived from the leading edge of ERG a-waves using an analytical expression derived from a quantitative model of phototransduction. The parameters obtained were the maximum cone P3 response (RmaxP3), sensitivity (S), and delay (t(d)). RESULTS: P3 response sensitivity is significantly higher in form-deprived eyes, at lower retinal irradiances. At higher flash intensities, in form-deprived eyes, P3 response sensitivity declines at a significantly greater rate, per unit increase in retinal irradiance, than in control eyes. Visual deprivation does not significantly affect RmaxP3 or t(d). CONCLUSIONS: Hypotheses to explain the altered cone photoreceptor sensitivity in form-deprived eyes are proposed. Changes in the biochemistry of phototransduction, or intrinsic geometric and physical attributes of photoreceptors or their waveguide modal properties, could account for the findings.