summary  Primary polydipsia is a common, relatively unknown, phenomenon that occurs in chronic psychiatric patients and can cause considerable morbidity and mortality. In this article we describe a 39-year-old female mentally retarded chronic psychotic, who suffered from intermittent hyponatremia due to polydipsia. The hyponatremia led to a periodic increase in restlessness and behavioural disturbances. Treatment with clozapine was found to improve the polydipsia, hyponatremia, and behavioural disturbances.