||Mitochondria are known to play a central role in cell death control. 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 GFP-tagged cytochrome c and morphological change of mitochondria within living HeLa cells during UV-induced apoptosis. We found that mitochondria underwent a modest degree of swelling in minutes after cytochrome c was released. 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 CCCP. In addition, we found evidence that cytochrome c was also released before mitochondrial swelling in apoptosis induced by other cell death treatments, including TNF and actinomycin D.