Th effects on ganglion cell light responses and spontaneous activity of neurotransmitter candidates, applied by nebulizer spray and iontophoresis, were studied in the isolated carp retina. ACh, GABA, and substance P had strong effects on the ganglion cells; dopamine and the amino acids aspartate, glutamate, and glycine and only weak effects. ACh and substance P exerted their actions even when synaptic transmission was blocked by cobalt chloride, suggesting postsynaptic receptors for those agents on the ganglion cell membrane. The 3 amino acids and dopamine do not appear to act directly on the ganglion cells. The pharmacological sensitivity of ganglion cells was correlated with their physiological response type. About three-quarters of ON/OFF and half of other transiently responding ganglion cells were excited by micromolar concentrations of cholinergic agonists; most ON-center sustained ganglion cells were insensitive. The light response of some of the ACh-sensitive cells could be suppressed by cholinergic antagonists. Substance P generally excited ganglion cells with an ON-component in their light response. GABA inhibited cells of all response types, but affected least the OFF-center tonic cells. In view of these observations, and of corroborating histological evidence, we propose that ACh, GABA, and substance P are neurotransmitters that are released by amacrine cells and affect receptors located on ganglion cells.