The gas hydrogen sulfide (H2S) is emerging as a novel regulator of important physiologic functions such as arterial diameter, blood flow and leukocyte adhesion. In addition, it may have antiinflammatory and antiapoptotic effects. H2S has recently attracted much interest as a potent vasorelaxative substance that may establish itself alongside another gaseous signal molecule, nitric oxide (NO). In contrast to NO, the major source of H2S in blood may be production by red blood cells or by vascular smooth muscle cells. H2S is produced from cysteine, involving the enzymes cystathionine beta-synthase and cystathionine gamma-lyase (CSE). The importance of CSE was recently demonstrated in a mouse lacking CSE which showed reduced H2S levels and developed hypertension and reduced endothelium-mediated vasorelaxation. These data establish H2S as a new and important biologic signal molecule and as a new regulator of vascular blood flow and blood pressure.