The role of endogenously produced H(2)S in mediating varied physiological effects in mammals has spurred enormous recent interest in understanding its biology and in exploiting its pharmacological potential. In these early days in the field of H(2)S signaling, large gaps exist in our understanding of its biological targets, its mechanisms of action and the regulation of its biogenesis and its clearance. Two branches within the sulfur metabolic pathway contribute to H(2)S production: (i) the reverse transsulfuration pathway in which two pyridoxal 5'-phosphate-dependent (PLP) enzymes, cystathionine β-synthase and cystathionine γ-lyase convert homocysteine successively to cystathionine and cysteine and (ii) a branch of the cysteine catabolic pathway which converts cysteine to mercaptopyruvate via a PLP-dependent cysteine aminotransferase and subsequently, to mercaptopyruvate sulfur transferase-bound persulfide from which H(2)S can be liberated. In this review, we present an overview of the kinetics of the H(2)S-generating reactions, compare the structures of the PLP-enzymes involved in its biogenesis and discuss strategies for their regulation. This article is part of a Special Issue entitled: Pyridoxal Phospate Enzymology.