The physiological role of nitric oxide (NO) in the maintenance of vascular tone, in synaptic transmission and in cellular defence is now firmly established. Recent evidence indicates that NO can also affect mitochondrial function. Here, we review findings indicating that NO — through its interaction with components of the electron-transport chain — might function not only as a physiological regulator of cell respiration, but also to augment the generation of reactive oxygen species by mitochondria, and thereby trigger mechanisms of cell survival or death.