Hydrogen sulfide (H2S), a messenger molecule generated by cystathionine gamma-lyase, acts as a physiologic vasorelaxant. Mechanisms whereby H2S signals have been elusive. We now show that H2S physiologically modifies cysteines in a large number of proteins by S-sulfhydration. About 10 to 25% of many liver proteins, including actin, tubulin, and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), are sulfhydrated under physiological conditions. Sulfhydration augments GAPDH activity and enhances actin polymerization. Sulfhydration thus appears to be a physiologic posttranslational modification for proteins.