Thiocyanate space was determined in 23 bilaterally nephrectomized rabbits in acute metabolic acidosis and alkalosis. Acid-base disturbances were induced by the infusion of 0.3 N HCI or 0.3 N NaOH solution intravenously with the rate of 1 ml/min for 40 to 60 minutes. The blood pressure was monitored throughout the experiment and no changes in blood pressure was confirmed. The following results were obtained. 1. In the saline infused control rabbits, PH was 7.385 with negligible change in pH after the infusion, SCN space was 23.6% of body weight. 2. In the metabolic acidosis group, pH dropped from 7.417 to 7 130 and SCN space was 22.8% of body weight and suggested a negligible change in the extracellular space volume. 3. In the metabolic alkalosis group, pH increased from 7.393 to 7.478 and SCN space was 25.7% of body weight which confirmed a significant increase in the extracellular space volume.