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To determine if a return to normonatremia is required for symptomatology to resolve in collapsed hypernatremic runners and if intravenous (IV) administration of an isotonic solution would adversely affect serum sodium concentration ([Na+]) in collapsed normonatremic runners.Observational study.2006 Comrades Marathon.103 collapsed runners.Final serum [Na+] upon discharge.58% of all collapsed runners were hypernatremic. Hypernatremic runners reported significantly more vomiting than normonatremic runners (79 versus 34%; P < 0.001). A significant decrease in serum [Na+] in hypernatremic collapsed runners occurred after the IV administration of either 1 L of 0.45% normal saline (150.5 ± 3.5 versus 148.0 ± 4.6; P < 0.05) or Ringers lactate solution (147.7 ± 2.2 versus 146.2 ± 2.1; P < 0.05). One liter of IV fluid administration caused an increase in plasma volume that was not significantly different between (1) hypernatremic runners receiving a hypotonic solution (13.5 ± 12.7%) and (2) normonatremic runners receiving an isotonic solution (15.6 ± 11.3%). The final serum [Na+] of hypernatremic runners was above the range for normonatremia upon discharge (>145 mmol/L).A return to normonatremia was not required for hypernatremic runners to “recover” and be discharged from the medical tent. Vomiting either aggravated and/or facilitated the development of hypernatremia. IV administration of 1 L of either (1) a hypotonic solution to hypernatremic runners or (2) an isotonic solution to both normonatremic and hypernatremic runners did not produce any adverse biochemical or cardiovascular changes and can therefore be considered a safe and effective treatment for collapsed runners if used in this context.