Mitochondrial function was examined in Jurkat cells undergoing Fas-mediated apoptosis. With succinate or ascorbate/tetramethylphenylenediamine as substrate, oxygen uptake by digitonin-permeabilized apoptotic mitochondria was greatly decreased as compared with control. Assessment of the function of the cytochrome c-cytochrome oxidase segment of the electron transport chain of apoptotic mitochondria showed that the activity of cytochrome oxidase appeared to be normal, but that of cytochrome c was greatly diminished. A death protease was found to participate in the events leading to the loss of cytochrome c activity, but the cytochrome did not seem to be extensively degraded during the course of apoptosis. Our results suggest that a rapid loss in mitochondrial function due at least in part to the inhibition or inactivation of cytochrome c is a potentially fatal component of the apoptosis program of Jurkat cells.