1. The effects of the sulphur containing amino acids, homocysteic acid, homocysteine sulphinic acid, cysteic acid and cysteine sulphinic acid on the release of [3H]-acetylcholine ([3H]-ACh) from the cholinergic amacrine cells of the rabbit retina were examined. 2. All the compounds stimulated the spontaneous resting release and abolished the light-evoked release of [3H]-ACh. Except for homocysteine sulphinic acid these actions occurred at concentrations that did not affect the erg b-wave amplitude, indicating a site of action at the inner retina. 3. N-methyl-D-aspartate (in Mg(2+)-containing medium) clearly blocked the effects of homocysteic acid and homocysteine sulphinic acid on the resting release of [3H]-ACh but had no effect on the actions of cysteic acid and cysteine sulphinic acid. 4. Since N-methyl-D-aspartate is an antagonist of the light-evoked endogenous bipolar cell transmitter released onto cholinergic cells, these results are consistent with the suggestion that homocysteic acid or homocysteine sulphinic acid may be a transmitter released from this subpopulation of bipolar cells. 5. The present experiments indicate the existence of excitatory amino acids that have closer pharmacological properties to a bipolar cell transmitter than glutamate but it remains to be seen whether homocysteic acid or homocysteine sulphinic acid occur in these particular bipolar cells.