We have obtained electroretinograms (ERGs) from rabbits and macaque monkeys after vitreal administration of 2-amino-4-phosphonobutyrate (APB), which selectively blocks light-responsiveness in retinal on-bipolar cells. Microelectrode recordings from central visual structures provided an independent measure of the drug effects. In rabbits, APB blocked the b-wave, a sustained corneal-positive potential, and a transient corneal-negative off-response. In monkeys, APB abolished the b-wave but had little or no effect on the d-wave. The d-wave could be eliminated, however, by the use of broadband blue stimuli. The results are discussed in light of recent hypotheses about the cellular origins of the ERG.