Beasley, KN, Lee, EC, McDermott, BP, Yamamoto, LM, Emmanuel, H, Casa, DJ, Armstrong, LE, Kraemer, WJ, and Maresh, CM. The effect of oral vs. intravenous rehydration on circulating myoglobin and creatine kinase. J Strength Cond Res 24(1): 60-67, 2010-Physical activity of significant intensity and duration may cause varying degrees of skeletal muscle damage, but it is unclear whether mode of rehydration will attenuate muscle tissue disruption caused by exercise in the heat. To examine the effects of the mode of rehydration on markers of muscle damage (myoglobin and creatine kinase [CK]), 11 healthy active men (age = 23 ± 4 years, body mass = 80.9 ± 3.9 kg, height = 180.5 ± 5.4 cm) completed 4 experimental trials consisting of an exercise dehydration protocol (to −4% of baseline body mass), followed by a rehydration period (oral, intravenous [IV], oral and IV combined, and ad libitum), and finishing with an intense exercise challenge that included treadmill running and sprinting and a box lifting protocol. During rehydration, subjects returned to −2% of baseline body mass unless completing the ad libitum trial during which they consumed fluids as thirst dictated. Myoglobin (Mb) and CK were measured during euhydrated rest. Post-exercise blood was drawn at 1 and 24 hours post exercise challenge for Mb and CK, respectively. Urine was collected during euhydrated rest and 1-hour post exercise challenge for measurement of Mb clearance. Mb concentrations increased significantly from pre (1.06 ± 0.20, 0.88 ± 0.07, 1.15 ± 0.25 and 0.92 ± 0.06 nmol·L−1) to post (1.52 ± 0.28, 1.44 ± 0.11, 1.71 ± 0.45 and 1.58 ± 0.39) for IV, oral, oral and IV combined, and ad libitum, respectively, but were not significantly different among trials. Serum CK concentrations remained within the normal physiological range for all trials. Thus, despite previous research that clearly indicates the benefit of ingesting fluids during exercise to attenuate muscle damage, there were no significant differences between the modes of rehydration on circulating Mb and CK.