DECREASED excretion of sodium and water and increased excretion of potassium during and following surgery have been reported by numerous investigators. Most of the reports have dealt with small groups of carefully selected cases. To collect further data on this subject, we have made studies on 38 consecutive major surgical cases in one hospital. A preliminary report on serum and urinary potassium1 includes an outline of the procedure and chemical methods used in this study. Data on urinary excretion of electrolytes and water before, during, and after surgery were sufficiently complete in 27 of the major surgical cases and in 2 of the minor surgical cases to warrant inclusion in this report (Table 1).