Hyponatremia is a metabolic disease of phocid and otariid seals. It is characterized by a gradual or sudden decrease in plasma sodium and chloride, and unpredictable changes in potassium. The condition is stress mediated, and appears with moult, malnutrition, and non-related diseases. Studies presently underway confirm stress disease interaction as the basis of hyponatremia. Diagnosis, therapy and control of the disorder will be discussed.