Forty adult patients undergoing non-cardiac major surgery were divided into 2 groups. Group A (n = 20) was anesthetized by balanced intravenous procaine anesthesia and group B (n = 20) by epidural block. Blood volume, urine output, potassium in plasma as well as urine, pH, glucose, aldosterone, cortisol and insulin were measured from 24 hours before operation to 48 hours after operation. Evident perioperative trend of hypokalemia in patients with normal renal function was most likely due to the following factors: potassium loss prior to operation, improper pre- or post-operative replacement of fluids, perioperative stress, increasing of blood insulin and urine potassium excretion. Our results run against to the general concept that it may not be necessary to supply potassium with 72 hours after operation.