Oxidative stress plays an important role in the development of tissue damage following transient focal cerebral ischaemia. Glutathione is a central component in the antioxidant defence of cells. We have previously shown a close association between mitochondrial glutathione loss and cell death following middle cerebral artery (MCA) occlusion. Glutathione monoethyl ester increases cellular glutathione and is particularly effective in increasing the mitochondrial pool. In the present investigation, we infused glutathione monoethyl ester into the third ventricle during 2 h of MCA occlusion and 48 h of reperfusion. Infarct size was reduced from 46% of the total ischaemic hemisphere in saline-treated animals to 16% following ester treatment. Thus, glutathione monoethyl ester provides neuroprotection following transient focal cerebral ischaemia.