We examined the separate and combined effects of hypohydration level and exercise intensity on aldosterone (ALD) and arginine vasopressin (AVP) responses during exercise-heat stress. Nine heat acclimated men performed 50 min of treadmill exercise in a warm room (30 degrees C dry bulb (DB), 50% relative humidity (RH) at 25%, 45% and 65% VO2max when euhydrated and when hypohydrated by 3% and 5% of body weight. Blood samples were drawn at rest and at 20 min of exercise. ALD and AVP increased (P < 0.05) in a graded manner with hypohydration level, and this effect persisted during exercise-heat stress. High intensity exercise produced greater ALD and AVP increases than low intensity exercise. ALD responses during exercise were independent of hypohydration level. AVP responses were closely related to osmolality (N = 6 of 7 subjects; r = 0.51 to r = 0.98; average r = 0.84) despite varying hydration, exercise intensity, or core temperature. We conclude that: 1) ALD and AVP increase in a graded manner with hypohydration, and this effect persists during exercise-heat stress; 2) ALD and AVP increases elicited by exercise are greater during high intensity than low intensity exercise; 3) Hypohydration and exercise intensity have additive effects on ALD: and 4) AVP responses are closely coupled to osmolality.