An application of the RAND computer model of fluid balance and electrolyte distribution in the human body (RM-4347) to solve a common problem of open-heart surgery: depletion of potassium in the blood serum interfering with the normal heartbeat. The potassium loss, or hypokalemia, has been attributed to preoperative diuretic therapy, digitalis, and hemodilution. The effects of extracorporeal hemodilution circulation, involving an extremely complex set of chemical thermodynamic relationships, were simulated, and the results checked against experiments with 16 dogs and observations of 5 patients undergoing heart-lung bypass. Hypokalemia is not prevented by simply adding potassium ion to the hemodiluent, nor by adding protein, but is prevented by adding both. (Presented at the Surgical Forum session of the American College of Surgeons, October 1970; to be published in [Surgical Forum].) 4 pp. Ref.