This chapter focuses on the measurement of γ-glutamyl transpeptidase and γ-glutamylcysteine synthetase activities in cells. There has been an increasing interest in glutathione (GSH) as it is the major nonprotein thiol in cells and is a preferred, if not specific, thiol substrate for several enzymes in xenobiotic metabolism and antioxidant defense. Under such stress conditions, cells must maintain glutathione and may even increase glutathione content above the steady state for maximum protection. The intraorgan transport and de novo synthesis of glutathione are two of the possible mechanisms for maintaining or increasing glutathione. The specificity of γGT toward γ-glutamyl compounds is quite broad and has allowed the development of several different assays. The most common assay uses _L-γ-glutamyl-p-nitroanilide with the product detected by absorbance spectrophotometry. Several studies have shown that γGT-mediated utilization of extracellular glutathione contributes to protection against oxidative injury. The importance of γGCS in maintaining glutathione in the face of the challenge of even endogenous generation of H₂O₂ has also been demonstrated.