Nitric oxide synthase (NOS) is the biosynthetic enzyme of the signaling molecule nitric oxide (NO). NO donors have been reported to modulate conductances in cell types throughout the retina, from photoreceptors to ganglion cells. Previously, NOS immunoreactivity has been reported in amacrine cells and cells within the ganglion cell layer. Here, we have examined the cellular localization of NOS in the retinas of salamander, goldfish, and catfish using both an affinity-purified antiserum to brain NOS and NADPH diaphorase (NADPHd) histochemistry. These markers indicate that an NOS-like enzyme is localized not only to presumptive amacrine cells but also, depending on the species, to photoreceptor ellipsoids, to somata within the ganglion cell layer, and to horizontal cells. In addition to these neurons, our results indicate that Muller cells, the radial glia of the retina, also contain an NOS- like enzyme. In support of this latter conclusion, cells morphologically similar to Muller cells were positive for NADPHd staining in all three species. In salamander, NOS-like immunoreactivity, NADPHd staining, and binding of anti-GFAP (a marker for glia) were localized to cells that were morphologically indistinguishable from Muller cells. In goldfish, reactivity to both anti-NOS and anti-vimentin (a marker for glia) colocalized to radial processes extending through the inner retina to the inner limiting membrane. These observations are the first to indicate the presence of an NOS-like enzyme in Muller cells and suggest that these glia could be a ready source of NO for target neurons throughout the retina.