The concentration of glutathione (GSH), the most abundant intracellular non-protein thiol and important antioxidant, declines with age and in some age-related diseases. The underlying mechanism, however, is not clear. The previous studies from this laboratory showed that the age-dependent decline in GSH content in Fisher 344 rats was associated with a down regulation of glutamate cysteine ligase (GCL), the rate-limiting enzyme in de novo GSH synthesis. Our recent studies further indicated that the activity and mRNA content of glutathione synthase, which catalyzes the second reaction in de novo GSH synthesis, was also decreased with age in some tissues. No age-associated change was observed in glutathione reductase and -glutamyl transpeptidase activities. It was also found that although GSH content declined with age in both male and female mice, male mice experienced more dramatic age-associated decline in many tissues/organs. Furthermore, we found that GSH content was significantly decreased in the red blood cells from male but not female Alzheimers disease patients, which was also associated with a decline in GCL activity. Finally, we showed that estrogen increased GSH content and GCL gene expression in the liver of both male and female mice. Taken together, our results suggest that 1) GCL plays a critical role in maintaining GSH homeostasis under both physiological and pathological conditions; 2) the decreased synthetic capacity rather than increased consumption underlies the age-associated decline in GSH content; 3) the different mechanisms may be involved in maintaining GSH homeostasis in male and female animals during the aging process.