In rare cases, drinking an extreme amount of water in a short time can be dangerous. It can cause the level of salt, or sodium, in your blood to drop too low. That's a condition called hyponatremia. It's very serious! Every hour, a healthy kidney at rest can excrete 800 milliliters, or 0.21 to 0.26 gallon, of water and therefore a person can drink water at a rate of about 800 milliliters per hour. Exercise does not midigate this concept!