We have investigated the effect of the centrifugation speed on the behavior of rat-liver mitochondria during isopycnic centrifugation in an isoosmotic medium. The gradient was made with a macromolecular compound, glycogen dissolved in 0.25 M aqueous sucrose. The distribution curves of several mitochondrial enzymes change when the centrifugation reaches a certain speed: they are shifted toward regions of lower density. The results are plausibly explained by supposing that the inner mitochondrial membrane becomes permeable to sucrose at high centrifugation speeds, and that the granules swell. The main causal agent of the phenomenon is the hydrostatic pressure the mitochondria are subjected to during centrifugation. Morphological observations show that mitochondria are markedly deteriorated when centrifuged at high speed in the glycogen gradient; they are swollen and the outer membrane is broken; also frequently, a large electron-dense granule is seen in the matrix near the inner mambrane.