Horizontal cell morphology was studied in the retina of the nocturnal owl-monkey,Aotus, and compared with that of its diurnal, close relative, the capuchin monkey,Cebus. Cells were initially labeled with DiI and the staining was later photoconverted in a stable precipitated using DAB as chromogen. The sizes of cell bodies, dendritic fields, and axon terminals, number of dendritic clusters, intercluster spacing, and intercone spacing were measured at increasing eccentricities. Two distinct morphological classes of horizontal cells were identified, which resembled those of H1 and H3 cells described in diurnal monkeys. A few examples of a third class, possibly corresponding to the H2 cells of diurnal monkeys, were labeled. Both H1 and H3 cells increased in size and had increasing numbers of dendritic clusters with eccentricity. H3 cells were larger and had a larger number of dendritic clusters than H1 cells. Owl-monkey H1 cells had larger dendritic fields than capuchin monkey H1 cells at all quadrants in the central and midperipheral retinal regions, but the difference disappeared in the far periphery. Owl-monkey and capuchin monkey H1 cells had about the same number of dendritic clusters across eccentricity. As owl-monkey H1 cells were larger than capuchin monkey H1 cells, the equal number of clusters in these two primates was due to the fact that they were more spaced in the owl-monkey cells. H1 intercluster distance closely matched intercone spacing for both the owl-monkey and capuchin monkey retinas. On the other hand, H3 intercluster distance was larger than intercone spacing in the retina of both primates. Owl-monkey H1 axon terminals had 2–3 times more knobs than capuchin monkey H1 axon terminals in spite of having about the same size and, consequently, knob density was 2–3 times higher for owl-monkey than capuchin monkey H1 axon terminals across all eccentricities. The differences observed between owl-monkey and capuchin monkey horizontal cells, regarding the morphology of their dendritic trees and axon terminals, may be related to the differences found in the cone-to-rod ratio in the retina of these two primates. They seem to represent retinal specializations to the nocturnal and diurnal life styles of the owl-monkey and capuchin monkey, respectively.