During apoptosis, cytochrome c is released from mitochondria to the cytosol to activate a caspase cascade, which commits the cell to the death process. It has been proposed that the release of cytochrome c is caused by a swelling of the mitochondrial matrix triggered by the apoptotic stimuli. To test this theory, we measured directly the dynamic re-distribution of green fluorescence protein (GFP)-tagged cytochrome c and morphological change of mitochondria within living HeLa cells during u.v.-induced apoptosis. We observed that mitochondria did not swell when cytochrome c was released from mitochondria to cytosol during apoptosis. Instead, mitochondria swelled to spherical shapes within 10 minutes of cytochrome c release. This finding strongly suggests that cytochrome c release in apoptosis was not caused by mitochondrial swelling. This conclusion was further supported in two separated experiments using an immunostaining method and carbonyl cyanide m-chlorophenyl-hydrazone (CCCP) treatment. In addition, we found evidence that cytochrome c was also released before mitochondrial swelling in apoptosis induced by other cell death-inducing treatments, including tumor necrosis factor (TNF) and actinomycin D.