The electrolyte isturbance hyponatremia (British hyponatraemia) exists in humans when the sodium (Natrium in Latin) concentration in the plasma falls below 135 mmol/L. At lower levels water intoxication may result, an urgently dangerous condition. Hyponatremia is an abnormality that can occur in isolation or, as most often is the case, as a complication of other medical illnesses. In the case of other mammals, particularly agricultural animals, different indications are relevant. The following refers to humans; an introduction to sodium deficiency in cattle is appended.