The bipolar cells of the vertebrate retina are the principal neuronal elements which transmit photoreceptor activity from the outer to the inner retina1. An important function of the bipolars is to segregate photoreceptor input into independent ON and OFF channels2,3 which are subserved, respectively, by the depolarizing and hyperpolarizing bipolar subtypes4-7. Ultra-structural8,9 and physiological 10 observations suggest that chemical neurotransmission is the predominant means of bipolar input to the inner retina. Both ON and OFF bipolars apparently release excitatory transmitters11-13. Histological studies with cytotoxic agents14-16 and physiological studies 17,18 indicate that third-order neurones have excitatory amino acid receptors. In ON-OFF amacrine and ganglion cells, which receive input from both bipolars12,19, ON and OFF excitation have a similar ionic basis 20, suggesting that the same transmitter may be released by both types of bipolars. We have now found that (±)cis-2,3-piperidine dicarboxylic acid (PDA)21, a new excitatory amino acid antagonist, blocks bipolar input to the inner retina and thus suggests that an excitatory amino acid is a bipolar cell transmitter.