The case of a 57-year-old male athlete who developed acute hyponatremia during participation in a 100 mile ultra-marathon is discussed. The initial presentation was one of rapid neurological deterioration and transient cardiovascular instability. Current theories on how hyponatremia develops in athletes are discussed. Treatment modalities, such as the use of hypertonic saline and the management of increased intracranial pressure, are addressed. With increasing interest in ultra-endurance events, the incidence of acute hyponatremia may increase. It is important that emergency physicians recognize this phenomenon and be familiar with the principles of acute intervention.