A relatively labile group of mitochondrial electrolytes has been analyzed. These materials were released when the mitochondria were "aged" by incubating them in unbuffered sucrose for 20-30 min. Most of the materials were released during the final 10 min, and this phase was marked by a gradual elevation of the pH of the medium from 6.1 to 7.1. In a representative study the following quantities ([mu]moles) of the various materials were obtained: potassium, 10.1; neutral phosphate (glycerylphosphorylethanol-amine), 2.6; total anionic phosphate, 6.7; inorganic phosphate, 1.3; and magnesium, 1.0. These water-soluble components appear to be contained in a portion of the mitochondrial substance that occupies 40-45% of the volume of the packed mitochondria; and in this part both the proportions, as listed above, and the concentrations are similar to those of the intact cell. Fresh mitochondria were extracted with perchloric acid to measure the concentrations of certain of the more labile phosphate compounds. One preparation, with 9.5 [mu]moles of potassium, contained 0.45 [mu]moles of adenosine triphosphate, 0.36 [mu]moles of adenosine diphosphate, and 0.33 [mu]moles of phosphoenolpyruvate.