Acetylcholine in the retina is synthesized and released by a sparse group of amacrine cells that symmetrically line both margins of the inner plexiform layer. The action of these neurons is at once very selective (they affect ganglion cells of some functional classes, but not others) and rather diffuse (the dependence of the acetylcholine-sensitive ganglion cells on cholinergic input is partial, and cholinergic synapses do not seem to affect the intrinsic structure of the signal transmitted by the ganglion cell). The most interesting aspect of these results is that one cannot yet fit the action of the cholinergic neurons to one of the physiological events thought to occur in the inner plexiform layer. Identification of the cholinergic cells thus seems to isolate a neural interaction that is not included among our concepts of the retina's internal working.