The goal of the present study was to develop a novel approach that facilitates the prescription of fluid therapy in patients with abnormal serum sodium concentration. The novel approach is based on a simple equation, derived from established principles on the distribution of sodium in body fluids, that estimates the impact of a unit dose, i.e., 1 l of any infusate on the patient's serum sodium concentration. In accordance with the equation, the expected change in the patient's serum sodium concentration in response to 1 l of any infusate (delta[Na+]s) is obtained by subtracting the sodium concentration of the patient's serum from the sodium concentration of the infusate, each expressed in mEq/l, and dividing the result by the patient's estimated total body water expressed in liters (adding 1 l to account for the volume of the infusate). The amount of the particular infusate to be administered over the course of any given time period can be easily computed by dividing the desired delta[Na+]s at the end of the period by the calculated delta[Na+]s effected by 1 l of the infusate. The utility and limitations of the proposed approach are presented. The novel equation is not a means for formulating therapy. Rather, it provides, simply and expeditiously, quantitative projections that can assist the physician in implementing the selected treatment plan for patients with dysnatremias.