In the frog retina, the inhibition exerted by the ON channel on the OFF channel was evidenced by the increase in transient ganglion cell OFF responses, when the ON channel was blocked by 2-amino-4-phosphonobutyrate (APB). Intraocular administration of the neurotoxic choline analog ethylcholine mustard arizidinium ion (ECMA) also provoked an increase in the number of spikes of transient ganglion cell OFF responses, without suppressing the ON responses. APB, when administrated after ECMA, abolished the ON responses, but did not modify the OFF responses already increased by ECMA. Neurons located in the inner part of the inner nuclear layer were histologically altered by the toxin, and choline acetyltransferase activity was significantly depressed in ECMA-treated retinas. A double immunostaining experiment showed that amacrine cells containing glycine bear muscarinic binding sites. These results confirm the participation of cholinergic neurons in the inhibition exerted by the ON retinal channel on the OFF retinal channel, and suggest the involvement of a cholinergic/glycinergic loop of amacrine cells in this mechanism.