T he effect of variations of medium pH and bicarbonate concentration on glutamine oxidation was studied in slices and mitochondria from dog renal cortex. Decreasing pH and bicarbonate concentration increased the rate of oxidation of glutamine-U-¹⁴C to ¹⁴CO₂ in both slices and mitochondria, an effect comparable to the acute stimulation of glutamine utilization produced by metabolic acidosis. Decreases in the concentration of glutamate and α-ketoglutarate, which accompany metabolic acidosis in the intact animal, also occurred in tissue slices when pH and [HCO₃^-] were lowered; decrease in α-ketoglutarate but not in glutamate content occurred in mitochondria under these conditions. Study of independent variations of medium pH and [HCO₃^-] showed that simultaneous changes in both pH and [HCO₃^-] produced a greater effect on glutamine metabolism than did change in either of these parameters alone.The rate of glutamine oxidation was also compared in tissue preparations from pairs of litter-mate dogs with chronic metabolic acidosis and alkalosis. No significant difference in the rate of glutamine oxidation was present in mitochondria from the two sets of animals. Slices from animals with chronic metabolic acidosis consistently oxidized glutamine at a more rapid rate than slices from alkalotic dogs both at high and at low concentrations of bicarbonate in the medium. We believe this difference is a result of the same mechanism which leads to the delayed increase in ammonium excretion during induction of metabolic acidosis.The close parallel between the effects demonstrated here and the changes in ammonium production and glutamine utilization in the intact animal with metabolic acidosis suggest that the observed in vitro changes accurately represent the operation of the physiologic mechanism by which acid-base changes regulate ammonium excretion. The similarity between the changes in glutamine oxidation observed in this study and those described previously for citrate suggests that one control mechanism affects the metabolism of both citrate and glutamine. Thus, we believe that the increase in citrate clearance in metabolic alkalosis and the increase in glutamine utilization and ammonium production in metabolic acidosis reflect the operation of the same underlying biochemical mechanism. This mechanism permits changes in pH and [HCO₃^-] in the cellular environment to regulate the rate of mitochondrial uptake and oxidation of several physiologically important substrates.