Mitochondrial intermembrane proteins including cytochrome c are known to activate caspases. Accordingly, a disruption of the mitochondrial membrane barrier function with release of cytochrome into the cytosol has been shown to precede caspase activation in a number of different models of apoptosis. Here, we addressed the question of whether caspases themselves can affect mitochondrial membrane function. Recombinant caspases were added to purified mitochondria and were found to affect the permeability of both mitochondrial membranes. Thus, caspases cause a dissipation of the mitochondrial inner transmembrane potential. In addition, caspases cause intermembrane proteins including cytochrome c and AIF (apoptosis-inducing factor) to be released through the outer mitochondrial membrane. These observations suggest that caspases and mitochondria can engage in a circular self-amplification loop. An increase in mitochondrial membrane permeability would cause the release of caspase activators, and caspases, once activated, would in turn increase the mitochondrial membrane permeability. Such a self-amplifying system could accelerate the apoptotic process and/or coordinate the apoptotic response between different mitochondria within the same cell.