Hydrogen sulfide (H(2)S) is a gaseous neuromodulator that can be synthesized by the transsulfuration enzymes cystathionine beta-synthase (CBS) and cystathionine gamma-lyase (CGL). In this study we examined H(2)S as a potential neuromodulator in vertebrate retina. CBS-like immunoreactivity (LI) was found in somas in the inner nuclear layer and as punctate staining in the inner and outer plexiform layers in the salamander retina. CGL-LI was most clearly characterized in salamander, where it was localized in Müller cells. Western blots indicated proteins with the correct molecular weights for both enzymes in both species for liver and cerebellum. Correct molecular weight proteins were identified for both CGL and CBS in salamander retina. The CBS antiserum did not recognize the correct molecular weight protein in mouse retina but the CGL antiserum recognized the correct molecular weight protein for mouse retina. Enzyme assays indicated both CGL and CBS enzyme activity in all three tissues in the salamander. There was good CBS activity in the liver and cerebellum of the mouse but no activity in the retina. CGL activity was clearly present only in the mouse liver, with only trace activity in the cerebellum and retina. In conclusion, both CBS and CGL are present in the amphibian retina, which suggests either a potential role for H(2)S as a gaseous neuromodulator in both neurons and glia in the retina or a requirement for cysteine and glutathione synthesis via the transsulfuration pathway as a defense against oxidative stress.