Rat-liver mitochondria isolated by differential centrifugation in isotonic sucrose were found to contain relatively constant amounts of sodium, potassium, magnesium and calcium. The particles were exposed to hypotonic surroundings and to lytic and spherulating agents, and as a result they were found to lose their potassium and to some extent their sodium while retaining magnesium and calcium. Potassium was chosen for closer study and the observation made that its retention by mitochondria in the face of unfavourable gradients was prevented by the action of 2:4-dinitrophenol (DNP) and by a brief incubation at 37 degrees C. The loss of cations under the latter conditions did not occur in the presence of a medium which enabled the tricarboxylic acid cycle to proceed. If DNP was also added the disappearance of potassium from the particles at 37 degrees C was unchecked. Incubating the bodies with potassium chloride at 2 and 37 degrees C led to an uptake of potassium wholly stable in the presence of DNP. After shaking at 37 degrees C with potassium glutamate, however, mitochondria, like untreated particles, contained only DNP-labile potassium. By varying the external concentration of potassium glutamate a net loss or net gain of the cation by the particles was demonstrated. If the protective medium was also present, loss of potassium was prevented but a net uptake could still be obtained. Sulphate and phosphate ions behaved like chloride, while a variety of other radicals, including carbonyl, carboxyl and sulphonyl groups, had a similar effect to that of glutamate. It was not possible to demonstrate an unequivocal uptake of sodium in any form by the particles. It is concluded that in isolated mitochondria as used in these experiments, maintenance of potassium concentration against an adverse gradient depends directly or indirectly on oxidative phosphorylation; that the successful uptake by these particles of potassium from the environment requires the presence of a suitable anion and that the bodies are able to treat sodium and potassium differently. Most of the potassium in isolated mitochondria is felt to be either ionized or potentially ionized, but it seems that at least one other form of the cation may exist, distinguished amongst other things by its behaviour in the presence of dinitrophenol. The relevance of the results to theories of mitochondrial structure is discussed. It is concluded that, in the light of present knowledge, the findings support the view that mitochondria possess some form of semipermeable membrane.