Background/Aim: Recently, we observed a progressive decline in the plasma ionized calcium (iCa) concentration after ingestion of Na-free beverage. Although both plasma pH and Na significantly correlated with iCa, the correlation of the latter was stronger than that of the former. Therefore, we explored the effect of Na on plasma iCa. Methods: Pooled human plasma was diluted with normal saline, 5% glucose, 0.9% KCl, or distilled water to examine the effect of plasma dilution on iCa. Also, to evaluate the effects of an isovolemic increase in ion concentration and osmolarity on plasma iCa, NaCl, KCl, and D-glucose were added to plasma. Electrolytes (Na<sup>+</sup>, K<sup>+</sup>, Cl<sup>–</sup>, iCa, HCO<sup>–</sup><sub>3</sub>), pH, and osmolarity were measured by using ion-selective electrodes. Results: In the plasma dilution study, although all solutions significantly decreased the iCa concentration, the decrease in iCa concentration in the normal saline treated solution was significantly lower than in others. When the plasma osmolarity was isovolemically increased, the iCa concentration significantly increased in parallel with the increase in osmolarity in the NaCl group (r = 0.604, p < 0.01, n = 50). In contrast, no significant change in the iCa concentration was seen in the other solutions. NaCl did not increase, but rather decreased the iCa concentration when added to a protein-free solution. Conclusion: The present study demonstrated that Na specifically affects plasma iCa independently of pH, presumably via modulating Ca binding to plasma proteins.