Recent advances in the study of the regulation of cell death by apoptosis suggest that changes in mitochondrial permeability frequently precede the development of morphological features such as chromatin condensation, phosphatidylserine inversion of the outer cell membrane and the activation of endonucleases. There is evidence that this permeability transition is associated with, and may be regulated by, changes in the intracellular redox potential. The role of the tripeptide thiol glutathione in the regulation of apoptosis-associated redox changes and the control of mitochondrial membrane permeability is reviewed in this article.