Water intoxication (also known as hyperhydration or water poisoning) is a potentially fatal disturbance in brain functions that results when the normal balance of electrolytes in the body is pushed outside of safe limits by over-consumption of water.[1] Normal, healthy (both physically and nutritionally) individuals have little to worry about accidentally consuming too much water. Nearly all deaths related to water intoxication in normal individuals have resulted either from water drinking contests, in which individuals attempt to consume more than 10 liters (3 gallons) of water over the course of just a few minutes, or long bouts of intensive exercise during which electrolytes are not properly replenished, yet massive amounts of fluid are still consumed.
the rest is: http://en.wikipedia.org/wiki/Water_intoxication