The purpose of this study was to examine the relationship between hyperventilation and increase in blood lactate during high-intensity constant-load exercise in heat and normal conditions. Seven male volunteers exercised for 10 min on a cycle ergometer at 80%(V) over dotO(2)max in heat (40 degrees C, 50% relative humidity: HT) and normal conditions (20 degrees C, 50% relative humidity: CON). Oxygen uptake, carbon dioxide output, ventilation, blood lactate and blood electrolytes (K(+), Na(+), Cl(-)) were measured in HT and CON. We found that ventilation was significantly higher during exercise in HT compared with CON (p < 0.05) and RER tends to be higher in HT than in CON. Blood lactate was significantly higher at 3 min during exercise in HT compared with CON (5.96 +/- 0.57 mEq.l(-1) 5.00 +/- 0.28 mEq.l(-1), p < 0.05). Change in strong ion difference [Delta SID = (Delta K(+) + Delta Na(+)) - (Delta Cl(-) + Delta La(-))], which affects Delta HCO(3)(-) in blood significantly, was lower at 5 min during exercise in HT compared with in CON (p < 0.05). These results suggest that hyperventilation during exercise in heat would induce lower HCO(3)(-) in blood and consequently would result in an increase in blood lactate at an earlier time during high-intensity exercise in heat. It was concluded that hyperventilation during short-term high-intensity exercise in heat is temporarily associated with an increase in blood lactate.