Acetoacetate, beta-hydroxybutyrate, pyruvate and lactate in whole blood, the acid-base balance according to Astrup, SGOT and SGPT, and the serum and urinary sodium and potassium were estimated in 10 fasting obese women. Fasting led metabolic acidosis due to a tenfold increase of acetoacetate and beta-hydroxybutyrate, while pyruvate and lactate had a tendency to decrease. Serum and urinary sodium and potassium decreased; A tendency to increased SGOT and SGPT was observed. Sodium bicarbonate administered in general since the 8th or 9th day of fasting led in 4 to 5 days to an earlier correction of blood pH, while pCO2, actual HCO3, standard HCO3, base excess, buffer base, pO2, Hb saturation and total CO2 reached the prefasting values only several days after blood pH normalisation. The administration of NaHCO3 corrected the serum level of sodium while its excretion remained low. Bicarbonate had no influence on the decreasing trend of serum potassium, but the urinary output of potassium was stabilized. It is suggested that in fasting subjects 1. the correction of the metabolic acidosis by NaHCO3 does not reduce the rate of lipid catabolism and does not lead to fluid retention and 2. liver damage in fasting may be prevented by blood pH normalisation.