Rat liver mitochondrial inner and outer membranes were subjected to the solubilizing effect of the nonionic detergent Triton X-100 under various conditions. After centrifugation, the supernatants (containing the solubilized fraction) and pellets were characterized chemically and/or ultrastructurally. The detergent seems to act by inducing a phase transition from membrane lamellae to mixed protein-lipid-detergent micelles. Different electron-microscopy patterns are shown by the inner membranes after treatment with different amounts of surfactant, whereas the corresponding images from outer membranes vary but slightly. Selective solubilization of various components is observed, especially in the case of the inner membrane. Some membrane lipids (e.g., cardiolipin) are totally solubilized at detergent concentrations when others, such as sphyngomyelin, remain in the membrane. Other inner-membrane components (flavins, cytochromes, coenzyme Q) show different solubilization patterns. This allows the selection of conditions for optimal solubilization of a given membrane component with some degree of selectivity. The influence of Triton X-100 on various mitochondrial inner-membrane activities was studied. The detergent seems to act especially through disruption of the topology of the functional complexes, although the activity of the individual enzymes appears to be preserved. Relatively simple enzyme activities, such as ATPase, are more or less solubilized according to the detergent concentration, whereas the more complex succinate-cytochrome c reductase activity practically disappears even at low Triton X-100 concentrations.