1. A method was developed for the assay of GSSG in heart tissue. 2. GSSG and total glutathione were measured in rat hearts perfused under a variety of conditions. About 2% of the total glutathione is present as GSSG. The concentrations of GSSG and GSH remained constant under all the conditions tested. 3. These results are discussed with reference to the equilibrium and rate of the glutathione reductase reaction in the cell. It is concluded that the enzyme reaction does not lie near equilibrium.
