The study aim was to examine the effects of mild hypertonic and isotonic hypohydration on endurance performance. Fifteen males completed three randomized 48-hr trials, where energy intake was standardized throughout. On day 1 participants completed a time to exhaustion (TTE1) after which they began one of three trials. On the control trial (CON) participants consumed adequate fluid to maintain euhydration. On the isotonic hypohydration trial (IH), participants consumed the same fluid as on CON, but at 0800 hr on day 3 they consumed 0.65 mg/kgof the diuretic furosemide. On the hypertonic hypohydration trial (HH), drinking fluids were restricted to 2 ml/kg/day for 48 hr. After 48 hr, the participants completed a second time to exhaustion (TTE2). Hydration and perceptual measures were assessed before each TTE, and thermoregulatory, cardiovascular, and perceptual measures were obtained during exercise. Before TTE2, dehydration on HH and IH was 1.9% and 2.0% of body mass, respectively. Plasma osmolality was higher on HD compared with CON and IH (286 ± 4, 296 ± 6, 286 ± 4; CON, HH, and IH, respectively; p < .01 vs. CON and IH) and plasma volume loss was only observed on IH (1.7 ± 6, –0.3 ± 6, –6.6 ± 4; CON, HH, and IH, respectively; p < .01 vs. CON and HH). Compared with CON endurance performance in TTE2 was reduced by 30% after HH (p < .05, d = 1.3) and 39% after IH (p < .01, d = 1.9). During TTE2, mean perceived exertion was greater in both HH and IH (p < .01 vs. CON). The reduction in endurance performance after HH was probably caused by differences in mood (vigor decreased 32% and fatigue increased 85%) and perceptual comfort (thirst increased 100%). The addition of cardiovascular strain is tentatively suggested as the cause for the greater reduction in endurance performance after IH (stroke volume decreased 7%). Isotonic hypohydration causes a greater reduction in endurance performance than hypertonic hypohydration evoked by prolonged fluid restriction.