An intrastriatal injection of NMDA produced an increase in glutathione to 152% of control values in mitochondria isolated from striatum at 1 h later. Total tissue glutathione was not changed. The mitochondrial increase was largely reversed by 2 h. Glutathione content was not significantly affected in mitochondria from a part of the cerebral cortex that did not exhibit damage following intrastriatal NMDA. Glutathione was similarly increased in mitochondria from both cortex and striatum at 1 h after a short period of forebrain ischemia, confirming our previous findings. The increases in mitochondrial glutathione developed shortly after accumulations of mitochondrial calcium that have been observed previously. Intravenous injection of cyclosporin A immediately following either the NMDA treatment or reversal of the ischemic period partially inhibited the increases in glutathione in mitochondria from the affected brain subregions. These studies provide evidence that early changes sensitive to cyclosporin A develop in mitochondria under pathological conditions in the intact brain. These glutathione increases are consistent with an induction of the mitochondrial permeability transition in the affected tissue.