Extracellular electrophysiological recordings were obtained from rabbit retinal ganglion cells in either a superfused eyecup or an in vivo preparation. Selective antagonists or agonists of serotonin at the 5-HT2 or 5-HT1A receptors were applied, and the changes in light-evoked and spontaneous activity were studied. Both 5-HT1A agonists and 5-HT2 antagonists reduced the ON-components of the light-evoked responses of all classes of brisk ganglion cell; spontaneous activity was reduced in these cells as well. These results suggest that the indoleamine-accumulating amacrine cells of the rabbit retina serve to facilitate the output of the depolarizing rod bipolar cell and thereby increase the efficacy of transmission between this and other cells in the rabbit retina, and that this process is mediated by 5-HT2 receptors. On the basis of the similarity of the actions of the 2 classes of drug studied, we hypothesize further that 5-HT1A receptors mediate an inhibitory process that serves to terminate the indoleamine-induced facilitation. This process may be located either in the bipolar terminal or presynaptic to it in the terminal of the putative indoleaminergic cells.