Polydipsia and water intoxication cause considerable morbidity and mortality in chronic psychiatric patients. Polydipsia in psychiatric patients has been described si nce the 1930s prior to the use of neuroleptics, but sys te matic study of the disorder has been done mostly in th e last two decades. There are many names for the dis order, which include psychogenic polydipsia (Bauer 1925), compulsive water drinking (Barlow and De Wardener 1959), psychosis, intermittent hyponatremia and polydipsia (PIP) syndrome (Vieweg et al. 1985) and self-induced water intoxication (Hobson and English 1963). The main purpose of this paper is to propose an hypothesis about the pathophysiology of the disorder and to reconceptualize some of the thinking about it.