This study examined the effects of hyperhydration, exercise-induced dehydration, and oral fluid replacement on physiological strain of horses during exercise-heat stress. On three occasions, six horses completed a 90-min exercise protocol (50% maximal O2 uptake, 34.5 degrees C, 48% relative humidity) divided into two 45-min periods (exercise I and exercise II) with a 15-min recovery between exercise bouts. In random order, horses received no fluid (NF), 10 liters of water (W), or a carbohydrate-electrolyte solution (CE) 2 h before exercise and between exercise bouts. Compared with NF, preexercise hyperhydration (W and CE) did not alter heart rate, cardiac output (Q), stroke volume (SV), core body temperature, sweating rate (SR), or sweating sensitivity during exercise I. In contrast, after exercise II, exercise-induced dehydration in NF (decrease in body mass: NF, 5.6 +/- 0.8%; W, 1.1 +/- 0.4%; CE, 1.0 +/- 0.2%) resulted in greater heat storage, with core body temperature approximately 1. 0 degrees C higher compared with W and CE. In exercise II, the greater thermal strain in NF was associated with significant (P < 0. 05) decreases in Q (10 +/- 2%), SV (9 +/- 3%), SR, and sweating sensitivity. We concluded that 1) preexercise hyperhydration provided no thermoregulatory advantage; 2) maintenance of euhydration by oral fluid replacement ( approximately 85% of sweat fluid loss) during exercise in the heat was reflected in higher Q, SV, and SR with decreased heat storage; and 3) W or an isotonic CE solution was equally effective in reducing physiological strain associated with exercise-induced dehydration and heat stress.