To determine the influence of hydration state upon circulatory controls, we studied four relatively fit subjects during duplicate 30-min cycle ergometer exercise bouts (55% VO2max) in euhydrated, hypohydrated, and hyperhydrated conditions. Ambient temperature was 35 degrees C. Hypohydration was achieved by 4 days of diuretic administration and resulted in a whole-body weight loss of 2.2 kg and a plasma volume decrease of approximately 700 ml. Hyperhydration was achieved by ADH administration plus ingestion of 2 liters water but caused only a minor increase volume. Hypohydration resulted in a significantly reduced cardiac output during exercise; this the result of a reduction in stroke volume of 17 ml.beat-1 without adequate elevation in heart rate. the internal temperature (Tes) threshold for cutaneous vasodilation was elevated by 0.42 degree C in hypohydrated conditions; but once vasodilation occurred, the slope of the arm blood flow:Tes relation was unchanged from control. Maximal arm blood flow was reduced by nearly 50% in hypohydration. These restrictions in cutaneous blood flow served to maintain an already compromised venous return, but due to the limitation of core-to-skin heat transfer, forced Tes to nearly 39 degrees C, significantly higher than in euhydrated conditions.