This study determined the effects of hypohydration on aerobic performance in compensable [evaporative cooling requirement (E(req)) < maximal evaporative cooling (E(max))] conditions of 10 degrees C [7 degrees C wet bulb globe temperature (WBGT)], 20 degrees C (16 degrees C WBGT), 30 degrees C (22 degrees C WBGT), and 40 degrees C (27 degrees C WBGT) ambient temperature (T(a)). Our hypothesis was that 4% hypohydration would impair aerobic performance to a greater extent with increasing heat stress. Thirty-two men [22 +/- 4 yr old, 45 +/- 8 ml.kg(-1).min(-1) peak O(2) uptake (Vo(2 peak))] were divided into four matched cohorts (n = 8) and tested at one of four T(a) in euhydrated (EU) and hypohydrated (HYPO, -4% body mass) conditions. Subjects completed 30 min of preload exercise (cycle ergometer, 50% Vo(2 peak)) followed by a 15 min self-paced time trial. Time-trial performance (total work, change from EU) was -3% (P = 0.1), -5% (P = 0.06), -12% (P < 0.05), and -23% (P < 0.05) in 10 degrees C, 20 degrees C, 30 degrees C, and 40 degrees C T(a), respectively. During preload exercise, skin temperature (T(sk)) increased by approximately 4 degrees C per 10 degrees C T(a), while core (rectal) temperature (T(re)) values were similar within EU and HYPO conditions across all T(a). A significant relationship (P < 0.05, r = 0.61) was found between T(sk) and the percent decrement in time-trial performance. During preload exercise, hypohydration generally blunted the increases in cardiac output and blood pressure while reducing blood volume over time in 30 degrees C and 40 degrees C T(a). Our conclusions are as follows: 1) hypohydration degrades aerobic performance to a greater extent with increasing heat stress; 2) when T(sk) is >29 degrees C, 4% hypohydration degrades aerobic performance by approximately 1.6% for each additional 1 degrees C T(sk); and 3) cardiovascular strain from high skin blood flow requirements combined with blood volume reductions induced by hypohydration is an important contributor to impaired performance.