Glutathione (GSH; gamma-glutamylcysteinylglycine) is ubiquitous in mammalian and other living cells. It has several important functions, including protection against oxidative stress. It is synthesized from its constituent amino acids by the consecutive actions of gamma-glutamylcysteine synthetase and GSH synthetase. gamma-Glutamylcysteine synthetase activity is modulated by its light subunit and by feedback inhibition of the end product, GSH. Treatment with an inhibitor, buthionine sulfoximine (BSO), of gamma-glutamylcysteine synthetase leads to decreased cellular GSH levels, and its application can provide a useful experimental model of GSH deficiency. Cellular levels of GSH may be increased by supplying substrates and GSH delivery compounds. Increasing cellular GSH may be therapeutically useful.