The identity of the retinal bipolar cell transmitter(s) is unknown although there is much indirect evidence that suggests it may be glutamate or a related compound. Some bipolar cells synapse onto cholinergic amacrine cells and in the rabbit retina acetylcholine (ACh) release is increased by light flashes and by the excitatory amino acids glutamate, aspartate and homocysteic acid (HCA). In the retina, the amino acid agonist N-methyl-D-aspartate (NMDA) is unusual in that it sometimes acts as an antagonist, and in the present experiments it blocked the light-evoked release of ACh by acting as an antagonist of the bipolar cell transmitter. However, NMDA did not block the actions of glutamate or aspartate on amacrine cell ACh release, a result that argues against either of these amino acids being the bipolar cell transmitter. On the other hand, the HCA evoked release of ACh was clearly antagonised by NMDA suggesting that HCA may be the bipolar cell transmitter released onto cholinergic amacrine cells. This suggestion is supported by the finding that the rabbit retina possesses HCA at a concentration of 0.8 nmol/g wet wt.