Synaptic N-Methyl D-Aspartate Receptor (NMDAR) activity is required for neuronal survival in the developing forebrain, and blockade of NMDAR activity induced cell death associated with oxidative stress [1-4]. We previously showed that synaptic NMDAR activity supports antioxidant defences in part by promoting the capacity of the thioredoxin-peroxiredoxin system [2]. Here we have investigated the regulation of another key antioxidant system: that centred on glutathione. We show that in rat neurons, Ca2+ signalling, particularly through NMDARs, is of critical importance to tune the capacity of the neuronal glutathione (GSH) system to the needs of an active neuron, and to guard against increased demand. Through a coordinated transcriptional program, synaptic activity enhances the capacity of the GSH system to combat oxidative insults. In particular, the transcriptional induction of GSH reductase activity and glutamyl-cysteine ligase (GCL) activity, the rate-determining enzyme in GSH biosynthesis, ensures that GSH levels are maintained in response to oxidative insults, despite basal rates of utilization being higher in active neurons. These changes to the supply side of the GSH pathway are complemented up by elevated rates of glutathione peroxidase activity, which together facilitate rapid reactive oxygen species (ROS) detoxification which would otherwise trigger Puma induction-dependent apoptosis. In vivo blockade of NMDAR activity in P7 rats by the i.p. administration of the antagonist MK-801 suppresses GCL expression and activity, leading to reduced GSH levels and neuronal death. The deleterious effects of low NMDAR activity can be suppressed by supplying neurons with a cell-permeable form of glutamyl-cysteine (GCEE), which bypasses the need for GCL activity and maintains GSH levels, preventing Puma induction and neuronal death. Thus, the NMDAR-dependence of the GSH biosynthetic pathway ensures that antioxidant defences can adapt to reflect activity levels, but render neurons particularly vulnerable to NMDAR blockade.