This study compared the effects of hypohydration on endurance exercise performance in temperate and cold air environments. On four occasions, 6 men and 2 women (age = 24 ± 6 yr, ht = 170 ± 6 cm, wt = 72.9 ± 11.1 kg, VO₂peak = 48 ± 9 ml/kg/min) were exposed to 3-hr of passive heat stress (45°C) in the early morning with (euhydration, EUH) or without (hypohydration, HYP, - 3% body mass) fluid replacement. Later in the day, subjects sat in a cold (2°C) or temperate (20°C) environment with minimal clothing for 1-hr before performing 30-min of cycle ergometry at 50%VO₂peak followed immediately by a 30-min performance time trial. Rectal (T_re) and mean skin temperatures (T_sk), heart rate (HR), and RPE measurements were made at regular intervals. Performance was assessed by the total amount of work (kJ) completed in the 30 min time trial. T_sk was significantly lower in the cold compared with the temperate trial, but there was no independent effect of hydration. T_re in both HYP trials was higher than EUH after 60-min of exercise, but the difference was only significant within the temperate trials (P<0.05). HR was significantly higher at 30-min within the temperate trial (HYP > EUH) and at 60-min within the cold trial (HYP > EUH) (P<0.05). RPE increased over time with no differences among trials. Total work performed during the 30-min time trial was not influenced by environment, but was less (P<0.05) for HYP than EUH in the temperate trials. The corresponding change in performance (EUH - HYP) was greater for temperate (-8%) than for cold (-3%) (P<0.05). These data demonstrate that 1) hypohydration impairs endurance exercise performance in temperate but not cold air, but 2) cold stress per se does not.