Inorganic phosphate concentrations and the values of energy charge were determined in the cytosolic and mitochondrial compartments of Ehrlich ascites tumour cells incubated with 5 mM glucose and/or 0.5 mM glutamine. The initial value of inorganic phosphate concentration in the cytosolic compartment decreased in the three incubation conditions assayed; this decrease was greater when glucose was present in the incubation medium. Mitochondrial inorganic phosphate concentration diminished in incubations with only glucose and raised in incubations with only glutamine.