Ten months following a severe head injury, a 26-year-old man developed a syndrome of inappropriate secretion of antidiuretic hormone (ADH) with a grand mal seizure occurring after an increased intake of non alcoholic beverages. The water loading test was negative but the sorbitol infusion test was positive. In the basal state, plasma concentration of ADH was inappropriate to plasma osmolality. ADH has also been measured during dynamic tests (infusions of sorbitol and of normal saline; complete and partial water restriction; propranolol and diphenylhydantoin treatments). In spite of the poor sensitivity of these measurements when ADH concentrations are low, our results indicate that the plasma osmolality threshold at which ADH secretion is inhibited was probably reset at a level lower than normal.