This study compared measured serum [Na(+)] (S([Na+]); brackets denote concentration) with that predicted by the Nguyen-Kurtz equation after manipulating ingested [Na(+)] and changes in body mass (DeltaBM) during prolonged running in the heat. Athletes (4 men, 4 women; 22-36 yr) ran for 2 h, followed by a run to exhaustion and 1-h recovery. During exercise and recovery, subjects drank a 6% carbohydrate solution without Na(+) (Na(+)0), 6% carbohydrate solution with 18 mmol/l Na(+) (Na(+)18), or 6% carbohydrate solution with 30 mmol/l Na(+) (Na(+)30) to maintain BM (0%DeltaBM), increase BM by 2%, or decrease BM by 2% or 4% in 12 separate trials. Net fluid, Na(+), and K(+) balance were measured to calculate the Nguyen-Kurtz predicted S([Na+]) for each trial. For all beverages, predicted and measured S([Na+]) were not significantly different during the 0%, -2%, and -4%DeltaBM trials (-0.2 +/- 0.2 mmol/l) but were significantly different during the +2%DeltaBM trials (-2.6 +/- 0.5 mmol/l). Overall, Na(+) consumption attenuated the decline in S([Na+]) (-2.0 +/- 0.5, -0.9 +/- 0.5, -0.5 +/- 0.5 mmol/l from pre- to postexperiment of the 0%DeltaBM trials for Na(+)30, Na(+)18, and Na(+)0, respectively) but the differences among beverages were not statistically significant. Beverage [Na(+)] did not affect performance; however, time to exhaustion was significantly shorter during the -4% (8 +/- 3 min) and -2% (14 +/- 3 min) vs. 0% (22 +/- 5 min) and +2% (26 +/- 6 min) DeltaBM trials. In conclusion, when athletes maintain or lose BM, changes in S([Na+]) can be accurately predicted by changes in the mass balance of fluid, Na(+), and K(+) during prolonged running in the heat.