Secretoneurin (SN) is a neuropeptide formed by endoproteolytic processing of secretogranin II (chromogranin C). Chromatographic analysis revealed that the human retina contains significant concentrations (14.2 fmol/mg wet weight) of this peptide. Its cellular localization in the retina was characterized by immunohistochemistry. SN-immunoreactive (IR) fibers showed a distinct distribution in central and peripheral retinal regions. Immunopositive somata were found in the ganglion cell layer and in the inner nuclear layer. The localization was similar to that of substance P. The physiological role of SN in the human retina is at present unknown. However, its presence in ganglion cells and/or amacrine cells suggests that it may play a role in visual processing.