Changes in the acid-base parameters of arterial and venous cattle blood (Figs. 1-3) as well as potassium and lactate concentration in plasma from venous blood (Figs. 4-5) were examined during anaerobic storage of the blood. The blood was stored at two different temperatures (21-24 degrees C and 0-4 degrees C) and was stabilized with either heparin alone or heparin and sodium fluoride. By lowering the storage temperature for heparinized blood the glycolysis processes were inhibited to such an extent that the values of the acid-base parameters did not change during the first 5-6 hours of the storage time. On storage at room temperature pH decreased slowly, but steadily. The values for pCO2 and BE respectively also increased and decreased slowly. On storage at room temperature the changes in the pO2 values of arterial blood (Fig. 2) decreased significantly while the venous pO2 values were nearly steady (Fig. 1). The changes in the pO2 values during storage are thought to be related to the oxygen-dissociation curve. The addition of sodium fluoride (Figs. 3-5) affects a number of the chemical processes of the blood to such an extent that the uncertainty on a single determination becomes very great. On storage of heparinized cattle blood at room temperature the potassium concentration in plasma (Fig. 5) can be determined with good certainty during the first 7 hours of the storage time. Lowering of the storage temperature or addition of sodium fluoride has the effect that the potassium concentration in plasma is changed essentially during storage.