Body water is distributed in three compartments - intracellular, extracellular and interstitial. In these water compartments, particles modelling the resulting osmolality are dissolved. Osmolality has been proved to be conspicuously stable in all these spaces due to equilibration on cellular membranes. Under clinical conditions, osmolality may be measured in serum or urine. The value of serum osmolality is governed particularly by the serum concentration of sodium, less by glucose, urea, albumin and some ions. Hyponatremia and hypernatremia represent most frequently dysectrolytemia, which may immediately endanger the child’s life. The paper presents a brief pathological basis of the changes in serum sodium concentration and a diagnostic-therapeutic approach to patients suffering from hyponatremia or hypernatremia. A short quiz is a part of the contribution aimed at continual medical education.

        Key words: hyponatremia, hypernatremia, diagnostics, therapy, continual education