The distribution of values of serum concentrations of sodium, chloride, potassium, and total carbon dioxide were studied in a large group of patients with congestive heart failure, and compared with the values in a smaller group of normal subjects used as controls. The depression of sodium and chloride concentration, and the elevation of total carbon dioxide content were evaluated in terms of renal function and mercurial therapy. In 11 of the cases with high serum contents of total carbon dioxide the acid-base equilibrium was precisely defined by measurement of pH in cutaneous whole blood and calculation of the pressure of carbon dioxide and of the concentration of buffer base.