Introduction Optimal performance during endurance exercise depends on a multitude of interrelated psychological and physiological varia- bles. The preservation of blood volume is one such factor as it is critical for maintaining an adequate supply of oxygen and nu- trients to the working muscles while facilitating the removal of metabolic waste [2]. The maintenance of sufficient plasma vol- ume becomes more crucial as significant quantities of metabolic heat are generated causing competition for blood flow between the skin and muscles. The onset of upright cycle exercise is char- acterized by a reduction in plasma volume that is intensity de- pendent and can approach 9% after 10 minutes of cycling at 70% peak V Ç O 2 [13]. Maintaining or minimizing this lost volume can be important for optimizing endurance performance, as isoos- motic hypovolemia has been shown to increase thermal and cir- culatorystrain [9]. Likewise, a significant improvement in endur- ance cycling has been shown to occur in participants whose plas- ma volumes were expanded above baseline prior to the com- mencement of exercise [18]. Initiation of very high intensity in- terval exercise has been shown to elicit expansion in plasma vol- ume within a 1±3 days [11,12]. This training induced hypervol- emic condition has been shown to persist until training is stopped (2±5). In contrast, others have not shown that acute hy- pervolemia improves endurance performance in the heat [16] or with trained cyclist with already high blood volumes [22]. Abstract The purpose of this study was to investigate the effects of an immediate pre-exercise, orally ingested, sodium load (164mEq Na + ) (IPOSL), equivalent to 10ml per kilogram of body weight, on plasma volume, endurance performance, and thermoregula- tion. Fourteen male participants consumed a nearly isotonic (255mOsm´kg ±1 )IPOSLandahypotonic(94mOsm´kg ±1 ), no-so- dium, placebo beverage (Pl) equivalent to 10ml´kg ±1 body weight in a randomized design. Subjects cycled at 70% of maxi- mal work rate, in a 21.0±23.3 8C lab, for 45 min while cardiovas- cular and thermoregulatory variables were measured. This was followed by a 15-min performance time trial. IPOSL and Pl inges- tion lead to a 3.1% expansion and a 4.7% reduction in resting baseline plasma volume, respectively. IPOSL maintained plasma volume during exercise to a greater extent than the Pl at 15 and 30, but not 45 min. There was a significant improvement (» 7.8%; p < 0.05) in time trial performance following IPOSL. No signifi- cant differences were observed for heart rate, core temperature, rate of perceived exertion or total body sweat rate (p > 0.05). In conclusion, IPOSL ingestion increased pre-exercise plasma vol- umes, maintained 15- and 30-min exercise plasma volumes, and improved an endurance performance time trial better than the Pl with no apparent compromise in thermoregulation. Key words Plasma volume ´ urine and plasma osmolality ´ core temperature ´ body weight loss Physiology & Biochemistry 182 Affiliation University of Utah, Salt Lake City, Utah, USA Correspondence M. Coles ´ California State University, Fresno, Department of Kinesiology ´ 5275N. Campus Dr. M/S SG28 ´ Fresno 93740 ´ California ´ USA ´ Phone: + 5592784166 ´ Fax: + 5592787010 ´ E-mail: mcoles@csufresno.edu Accepted after revision: February 6, 2004 Bibliography Int J Sports Med 2005; 26: 182±187  Georg Thieme Verlag KG ´ Stuttgart ´ New York ´ DOI 10.1055/s-2004-820989 ´ Published online September 10, 2004 ´ ISSN 0172-4622 M. G. Coles M. J. Luetkemeier Sodium-Facilitated Hypervolemia, Endurance Performance, and Thermoregulation