The primate electroretinogram (ERG) recorded at the cornea in response to fast flickering light is thought to reflect primarily the cone photoreceptor potential. We investigated the origin of the photopic 33-Hz corneal flicker ERG to square-wave and photostrobe flashes by recording in the monkey before and after blocking postsynaptic responses with intravitreal injections of 2-amino-4-phosphonobutyric acid and/or cis-2,3-piperidiendicarboxylic acid or sodium aspartate. Blocking postsynaptic ON or OFF responses produced effects on the timing and the waveform of the 33-Hz flicker ERG similar to changes in the b and the d waves in the corneal single-flash ERG. When all the ERG waves of postsynaptic origin in the flash ERG were abolished the flicker response was greatly suppressed, suggesting the postsynaptic cells producing the b and the d waves make major contributions to the photopic fast flicker ERG.