The effect of acid-base balance on the oxidation and utilization of citrate and other organic acids has been studied in tissue slices and isolated kidney mitochondria. The results show that: 1) With bicarbonate-buffered media, citrate oxidation and utilization are inhibited in slices of renal cortex and in kidney mitochondria when [HCO(3) (-)] and pH are increased within the physiologic range (pH 7.0 to 7.8; 10 to 60 mumoles HCO(3) (-) per ml). When phosphate or Tris buffers are used, no comparable effect on citrate oxidation occurs when pH is varied. 2) This effect is not demonstrable in heart or liver slices when a physiologic buffer is used. 3) alpha-Ketoglutarate utilization is inhibited in slices of renal cortex under similar conditions. Pyruvate and L-malate utilization are not inhibited in slices or mitochondria. 4) Citrate content in slices of renal cortex incubated with a high [HCO(3) (-)] is considerably greater than the concentration found with a low [HCO(3) (-)] in the medium. This effect is not duplicated by pH change in a nonbicarbonate buffer system. In mitochondria citrate content is also increased markedly at high bicarbonate concentrations. 5) The kinetic characteristics of the inhibition of citrate oxidation are those of a competitive type of inhibition. 6) When pH was varied with a constant [HCO(3) (-)] in the media, citrate oxidation was inhibited by increasing pH in slices of renal cortex but not in mitochondria. On the other hand, when [HCO(3) (-)] was increased without change in pH, no decrease in citrate oxidation occurred in slices, but a marked inhibitory effect was found when mitochondria were used. From a comparison of these results with those previously obtained in intact animal experiments, we conclude that the inhibition of citrate oxidation caused by increasing pH and [HCO(3) (-)] in slices of renal cortex and kidney mitochondria is an in vitro representation of the inhibition of citrate reabsorption in the nephron that occurs in metabolic alkalosis. Thus, citrate clearance increases in metabolic alkalosis because of inhibition of oxidation of reabsorbed citrate within cells of the renal tubules. This inhibition is the result of an inhibitory effect of bicarbonate ion on citrate oxidation in mitochondria.