Rat 3-mercaptopyruvate sulfurtransferase (MPST) is a 32808Da simple protein. Cys(247) is a catalytic site, and Cys(154) and Cys(263) are on the enzyme surface. MPST is found in all tissues, particularly in the kidneys, although the localization of its activity differs in each tissue. In this review, four functions of MPST are reviewed: (i) antioxidative function: Cys(247) is redox-sensitive and serves as a redox-sensing switch. It is oxidized to cysteine sulfenate, which has a low redox potential, upon which the enzyme is inactivated. Then, reduced thioredoxin (Trx) with a reducing system (Trx reductase and NADPH) reduces the sulfenate to restore activity; meanwhile, Cys(154) and Cys(263) form an intermolecular disulfide bond, which serves as another redox-sensing switch. Consequently, Trx specifically cleaves the intermolecular disulfide bond by converting it from the inactive form (dimer) to the active form (monomer). (ii) Hydrogen sulfide and polysulfide production: hydrogen sulfide is produced via reduction of the persulfurated sulfur-acceptor substrate by reduced Trx or Trx with a reducing system; as an alternative process, stable polysulfurated or persulfurated Cys(247) as a reaction intermediate is reduced by Trx with a reducing system to release hydrogen sulfide and polysulfides. (iii) Possible sulfur oxide production: sulfur oxides (SO, SO2 and SO3) can be produced in the redox cycle of sulfane sulfur formed at the catalytic site Cys(247) (Cys-SO-, Cys-SO2- and Cys-SO3-) as reaction intermediates and released by reduced Trx or Trx with a reducing system. (iv) Possible anxiolytic-like effects: MPST-knockout mice exhibited anxiolytic-like effects.