Abstract Although serotonin (5-HT) is found in the mammalian retina only at low levels, considerable evidence suggests that it plays a role in visual processing. Pharmacological experiments indicate that numerous receptors for 5-HT are present in the mammalian retina. One of these is the ionotropic 5-HT 3 receptor. So far, two subunits for this receptor have been identified in the nervous system, 5-HT 3A and 5-HT 3B. Co-expression of these subunits in Xenopus oocytes is sufficient to reconstitute native 5-HT 3 receptor properties. Thus, it is believed that a native neuronal 5-HT 3 receptor is multimeric similar to the related acetylcholine receptor family. To determine whether this receptor is expressed in the mammalian retina, we first performed reverse transcription polymerase chain reaction and first demonstrated the presence of transcripts for both the 5-HT 3A and 5-HT 3B receptor subunits. Then using a well-characterized polyclonal antiserum against the 5-HT 3A receptor subunit, we demonstrated 5-HT 3A receptor immunoreactivity (IR) in the rabbit, rat, and human retina. This IR was localized specifically to the rod photoreceptor terminals in all three species, suggesting that this receptor may modulate the rod signaling pathway by controlling the output at the rod terminals.