CASA, D. J., C. M. MARESH, L. E. ARMSTRONG, S. A. KAVOURAS, J. A. HERRERA, F. T. HACKER, JR., N. R. KEITH, and T. A. ELLIOTT. Med. Sci. Sports Exerc., Vol. 32, No. 1, pp. 124–133, 2000. The purpose of this study was to assess whether a brief period (20 min) of intravenous (IV) fluid rehydration versus oral rehydration differentially affects cardiovascular, thermoregulatory, and performance factors during exhaustive exercise in the heat. Following dehydration (−4% of body weight), eight nonacclimated highly trained cyclists (age = 23.5 ± 1.2 yr; V̇O2peak = 61.4 ± 0.8 mL·kg·min−1; body fat = 13.5 ± 0.6%) rehydrated and then cycled at 70% V̇O2peak to exhaustion in 37°C. Rehydration (randomized, cross-over design) included: 1) CONTROL (no fluid), 2) DRINK (oral rehydration, 0.45% NaCl) equal to 50% of prior dehydration, and 3) IV (intravenous rehydration, 0.45% NaCl), equal to 50% of prior dehydration. Thus, in the DRINK and IV treatments subjects began exercise (EX) at −2% of body weight. Exercise time to exhaustion was not different (P = 0.07) between DRINK (34.9 ± 4 min) and IV (29.5 ± 3.5 min), but both were significantly (P P Thus, despite no statistically significant performance differences between DRINK and IV, it appears that certain physiological parameters were better maintained in the DRINK trial, and the trend toward performance differences may be important to elite athletes.