The respiratory activity and ionic balance of mitochondria isolated from carrot and beet tissues by differential centrifugation have been studied. The oxygen uptake of the mitochondria with different substrates was investigated. The mitochondria hold both cations and anions in concentrations greater than those in the supernatant. Experiments on the time of adjustment to a changed concentration of chloride in the supernatant solution have been used to calculate the diffusion constant of salt in the particle. On the assumption that most of the resistance to diffusion is in the surface membrane (thickness about 200 A), the apparent diffusion constant of chloride in the membrane was shown to be of the order of 10-1:> cm~/sec. This agrees with that found for heart muscle sarcosomes under similar conditions and is of the order expected in a !ipo-protein membrane. The concentrations of mobile cations (Na + and K +) in the mitochondria are considerably greater than those in the supernatant. It is suggested that the internal concentrations are largely due to a Donnan equilibrium based on the immobile anions of the particle. Since no simple Donnan equilibrium will account for the simultaneous concentration of both mobile cations and mobile anions, it is suggested that the mobile anions might be accumulated by an accumulatory mechanism. The anion concentration difference between the inside and outside of the particles is of the order of magnitude to be expected if the electron carrier of� respiration were acting as the anion carrier of accumulation. The results are therefore not inconsistent with the em'lier hypotheses for the interdependence of the two processes. The results support the hypothesis that mitochondria are probably involved in electrolyte accumulation in plant cells and in secretion in animal cells such as those of the gastric mucosa.