Hydrogen sulfide (H₂S) is a signaling molecule in various tissues and organs. We demonstrated H₂S as a neuromodulator in the brain with its producing enzyme cystathionine beta-synthase (CBS) in 1996. In the following year, we demonstrated H₂S as a vascular smooth muscle relaxant in synergy with nitric oxide (NO) with cystathionine gamma-lyase (CSE) as another producing enzyme. We also found the cytoprotective effect of H₂S; H₂S protects neurons from oxidative stress. In addition to CBS and CSE we found 3-mercaptopyruvate sulfurtransferase (3MST) along with cysteine aminotransferase (CAT) produces H₂S in the brain, retina and vascular endothelium. Although 3MST requires a reducing substance to produce H₂S, the endogenous reducing substance has not been identified. We found that thioredoxin and dihydrolipoic acid (DHLA) are the endogenous reducing substances to make 3MST produce H₂S. H₂S production by 3MST is also regulated by Ca²⁺; the production is maximal in the absence of Ca²⁺ and is suppressed by Ca²⁺ in a concentration-dependent manner. Calmodulin is not involved in this regulation by Ca²⁺. In the retina H₂S suppresses Ca²⁺ influx and maintains the intracellular Ca²⁺ concentrations at low levels. H₂S protects retinal neurons from light-induced degeneration.