Fluid-electrolyte balance is regulated within a narrow range and disturbances in this system are unusual in animals and humans. Studies from the preneuroleptic era to date suggest that up to 25 percent of patients with schizophrenia have polydipsia, suggesting that it is related to the pathophysiology of the psychoses. Polydipsia and the related phenomenon of hyponatremia cause considerable mortality and morbidity. Prevalence studies are limited by imprecise measures available at present. The treatment was limiting water intake when patients reached critical levels of water retention, which however did not improve polydipsia. Recent case reports and open studies have shown that clozapine improves both polydipsia and water retention. The response occurs at low doses and is not related to improvement in psychosis. This may not be applicable to all patients and better understanding of the pathophysiology of polydipsia-hyponatremia would lead to more empirically derived treatments.