A 20-year-old military recruit suffered a generalized tonic-clonic seizure following 9 hours of moderate activity in a hot, humid environment. He had drunk at least 5.8 L of plain water before the seizure, and laboratory studies revealed that his serum sodium concentration was 113 mmol/L. Overconsumption of fluids during exercise may precipitate acute hyponatremia, a potentially life-threatening medical condition. Prompt correction of serum sodium in acute exertional hyponatremia is important to reduce the risk of permanent neurologic sequelae or death. Recommendations for prevention include ingesting the correct amount of fluid for the activity (the most important method) and consuming adequate salt through diet or beverage.