This study investigated the effects of progressive mild dehydration during cycling on whole body substrate oxidation and skeletal muscle metabolism in recreationally active men. Subjects (n=9) cycled for 120 min at ~65% VO2peak (22.7°C, 32% RH) with water to replace sweat losses (HYD) or without fluid (DEH). Blood samples were taken at rest and every 20 min and muscle biopsies were sampled at rest, 40, 80, and 120 min of exercise. Subjects lost 0.8%, 1.8% and 2.7% BM after 40, 80 and 120 min of cycling in the DEH trial while sweat loss was not significantly different between trials. HR was greater in the DEH trial from 60-120 min and Tc was greater from 75-120 min. RPE was higher in the DEH trial from 30-120 min. There were no differences in VO2, RER, total CHO oxidation (HYD, 312 ± 9 vs. DEH, 307 ± 10 g) or sweat rate between trials. Blood [La] was significantly greater in the DEH trial from 20-120 min with no difference in plasma [FFA] or [EPI]. Glycogenolysis was significantly greater (24%) over the entire DEH vs. HYD trial (433 ± 44 vs. 349 ± 27 mmol/kg dm). In conclusion, dehydration of <2% BM elevated physiological parameters and perceived exertion, as well as muscle glycogenolysis during exercise without affecting whole body carbohydrate oxidation.nhttp://www.ncbi.nlm.nih.gov/pubmed/23114793n