This study assessed the need to replace sodium in endurance exercise ≤6 h in duration by comparing responses to fluid replacement with water, saline (25 mmol·l−1), or no fluid. Eight subjects (five male, three female) participated in three 6-h exercise trials on an electrically braked cycle ergometer at 55% V̇O2max, at 30°C and 50% r.h. In the water (W) and saline (S) trials, sufficient fluid was ingested to balance sweat and urinary fluid losses, while in the third trial, no fluid (NF) was ingested. Plasma sodium < 130 mmol·1−1 was a criterion for trial termination. In the NF trial, heart rate, rectal temperature, plasma sodium, plasma aldosterone, and rating of perceived exertion were all significantly higher (P < 0.001) than during W and S, whereas plasma volume was lower (P < 0.001). On average, subjects terminated this trial 1.5 h prior to its scheduled completion, having lost 6.4% body weight. In contrast, no significant differences between fluid replacement with W or S were detected, although the effect of time on all aforementioned variables was highly significant (P < 0.001). Saline intake was not associated with significantly higher plasma sodium during exercise than was water intake: plasma sodium decreased significantly during both W (to 135.5 ± 0.5 mmol·l−1) and S (to 137.3 ± 0.7 mmol·l−1). No subject had to terminate a trial based on plasma sodium ≤ 130 mmol·−1. The results of this study reiterate the importance of adequate fluid intake during endurance exercise and indicate that a need for sodium replacement would be unexpected in exercise of less than 6 h duration, assuming plasma sodium is normal at the start of exercise. Moreover, the amount of sodium in commercial beverages is inadequate to prevent a decrease in plasma sodium.