Acute hyponatremia is a serious condition, which poses major challenges. Of particular importance is what determines plasma sodium concentration (plasma [Na⁺]). Edelman introduced an explicit model to describe plasma [Na⁺] in a population as [Na⁺] = •(exchangeable Na⁺ + exchangeable K⁺)/(total body water) - . Evidence for the clinical utility of the model in the individual and in acute hyponatremia is sparse. We therefore investigated whether the actual measured plasma [Na⁺] was equal to the value estimated in a porcine model of hyponatremia. Plasma [Na⁺] was estimated from in vivo-determined balances of water, sodium and potassium according to a conversion of Edelman's equation. Acute hyponatremia was induced with desmopressin acetate (DDAVP) and infusion of a 2.5% glucose solution in anesthetized pigs. During 480 minutes, plasma [Na⁺] and osmolality were reduced from 136 mmol/l (SD=2) to 120 mmol/l (SD=3) and from 284 mOsm/kg (SD=4) to 252 mOsm/kg (SD=5), respectively. First, Edelman's model, which takes into account sodium and potassium, fits plasma [Na⁺] significantly better than dilution alone. Second, common = 1.33 (SD=0.08) and = -13.04 mmol/l (SD=7.68) for all pigs explain the plasma [Na⁺] in the individual animal. Third, measured exchangeable Na⁺ and calculated exchangeable Na⁺+K⁺ per weight in the pigs are close to Edelman's findings in humans, whereby the methods are cross-validated. In conclusion, plasma [Na⁺] can be explained in the individual animal by external balances according to Edelman's construct in acute hyponatremia.