This work deals with the selective demineralization of natural saltwaters by electrodialysis on specific membranes with the aim of preparing an "isotonic" water of blood plasma, i.e., of the same saline composition as blood, with potential applications in the field of health. Preliminary experiences conducted with experimental saltwater consisting of pure water and the ions Na+, Mg+, Ca2+, Cl- and SO42- showed that cationic membranes CMX-S and anionic membranes ACS allowed a fast reduction of the ratio of Monovalent/divalent ions, whereas divalent ion concentration level is weakly lowered. With underground saltwater, the purpose of reducing five times the monovalent ion concentration is easily reached. The decrease in sodium chloride contents of the solution introduced in the concentrate compartment as well as the decrease of initial current intensity improve selectivity, but increasing the time of treatment is needed to reach satisfactory sodium contents. In conclusion, the study shows that electrodialysis allows one to produce isotonic water using a simpler operating procedure than dialfitration on nanofiltration membranes.