It has been reported that dextrans diminish the intermembrane space of mitochondria, increase the number of contact sites between the inner and the outer mitochondrial membranes, decrease the outer membrane permeability to adenosine 5(')-diphosphate, and change the kinetic properties of mitochondrial kinases. In the present work the influence of dextran M40 (5% w/v) on the oxidoreductase activities of the inner and outer membranes of mitochondria, the interaction of cytochrome c with mitochondrial membranes, and the light scattering by rat liver mitochondria were studied. No influence of dextran on the release of cytochrome c from mitochondria or its interaction with mitochondrial membranes was observed. Decreases in the NADH-oxidase (to 80+/-2% of the control), NADH-cytochrome c reductase (to 26+/-2%), succinate-cytochrome c reductase (to 70+/-5%), and NADH-ferricyanide reductase (to 75+/-3%) activities induced by dextran, which may be due to the mitochondrial aggregation, were observed. The formation of aggregates was registered by light scattering, confirmed by light microscopy, and explained within the framework of the Gouy-Chapman theory of the electrical double layer. The observed mitochondrial aggregation seems to be useful also for understanding the mechanisms of mitochondrial condensation and perinuclear clustering during apoptosis.