This study was designed to assess the effects of hydration, acclimation, environment, and exercise on plasma levels of renin and aldosterone. Sixteen subjects exercised (1.34 m X s-1), both pre- and postacclimation, when euhydrated or hypohydrated (-5% of body wt) in a comfortable (20 degrees C, rh = 40%), hot-wet (35 degrees C, rh = 79%), or hot-dry (49 degrees C, rh = 20%) environment. Although light exercise in a thermoneutral environment had no effects on plasma levels of renin activity (PRA) or aldosterone (ALD), exercise in both hot environments resulted in significantly increased levels of both. Increments in both PRA and ALD were greater when hypohydrated, and PRA effects were significantly moderated by heat acclimation in both the euhydration and hypohydration experiments. Although PRA and ALD responses were generally correlated, acclimation did not consistently attenuate ALD increments. We concluded that hydration state, acclimation level, and environmental conditions all affected the responses of PRA and ALD to light exercise.