The proposed study will devise a quantitative model to estimate the amount of water an individual would need to ingest in order to lower the 24 h mean urine osmolality to a level below plasma (~285 mOsm/Kg). This osmolality goal is chosen because the 24h median urine osmolality of humans is ordinarily ~753 mOsm/Kg, much greater than 285 mosm/Kg (6, 7). In other words, normal humans are usually anti-diuretic during waking hours and while asleep. Median 24h urine volume is ~1225 ml (range 1051 - 2270). In temperate climates the insensible losses of water in sweat, respiration and stool are nearly balanced by the water ingested in solid and semi-solid foods and derived from metabolism. Thus, daily urine volume measured upon arising in the morning is a reasonably good indicator of the volume of fluids drunk over the preceding 24 h.