Cellular glutathione peroxidase (c-GPx) is a member of a family of GPx enzymes whose function is to detoxify peroxides in the cell, thus playing a critical role in protecting the cell from free radical damage, particularly lipid peroxidation. This assay provides an indirect measure of the activity of c-GPx. Oxidized glutathione (GSSG), produced upon reduction of an organic peroxide by c-GPx, is recycled to its reduced state by the enzyme glutathione reductase (GPx). The oxidation of NADPH to NADP+ is accompanied by a decrease in absorbance at 340 nm, thus providing a spectrophotometric means for monitoring GPx enzymes activity. The rate decrease in the A340 is directly proportional to the GPx activity in the sample.