Polydipsia can be defined as an impulsive behavior leading to absorption of large amounts of water (4 to 20 litres a day), without any underlying organic disease. Its prevalence in a population of chronic psychiatric patients can be as high as 6 to 17%. Schizophrenia represents 80% of cases reported. Some patients with polydipsia may develop hyponatremia, leading to a PIP syndrome (Polydipsia intermittent hyponatremia and psychosis). Hyponatremia or water intoxication appears when three conditions are present: an abnormal regulation of thirst, an inappropriate ADH secretion and/or an excessive renal sensitivity to ADH, with an increased sensitivity of the central nervous system to hyponatremia. The clinician must first identify patients at risk to develop water intoxication and start treatment before any severe physical complication occurs. Pharmacological treatments aiming at an increase of renal free-water excretion--do not show a constant efficacy in the correction of hyponatremia, they have no action on polydipsia. The new atypical neuroleptics such as clozapine and risperidone seem to open new perspectives in the treatment of polydipsia. Controlled studies should be performed in this field.