To determine the effect of hydration on cardiovascular drift (i.e.; increased heart rate and reduced stroke volume) during exercise in a 21 degrees C environment, nine subjects were studied while cycling at 65% of peak oxygen consumption when euhydrated and while still hypohydrate, following exercise-induced dehydration and a 2 h rest/rehydration period. Subjects dehydrated by exercising in the heat (32 degrees C) until body weight was reduced 2.5%. On two separate occasions following exercise, subjects either received no fluid or ingested a volume of water equal to 100% of the fluid lost during exercise. Following the 2 h rest/rehydration period, 65 +/- 6% of the ingested water was retained and thus the subjects were hypohydrated by 0.9 +/- 0.1%, compared to being hypohydrated by 2.8 +/- 0.1% when no fluid was ingested. Despite these differences in whole body hydration, blood volume during exercise remained at euhydrated levels when hypohydrated by 0.9% and 2.8%. However, the degree of cardiovascular drift was graded in proportion to hypohydration. Compared to the responses when euhydrated, heart rate was elevated 10 +/- 2 and 18 +/- 2 bt/min, whereas stroke volume was reduced 9 +/- 3 and 18 +/- 2 ml/bt, respectively, when hypohydrated by 0.9% and 2.8% during the water and no fluid trials (p < 0.05 for all comparisons). These observations indicate that cardiovascular drift during exercise in a 21 degrees C environment is graded in proportion to hydration and, under these conditions, not due to reductions in blood volume.