Numerous investigators have suggested that dehydration may alter carbohydrate metabolism in muscle. Theoretically, altered glycolytic flux in exercising muscle could change the rate at which lactate accumulates in the blood and thus alter the onset of blood lactate accumulation. The purpose of this study was to investigate the effects of acute dehydration on blood lactate accumulation during incremental exercise. Six trained male subjects performed an incremental test to exhaustion on a cycle ergometer during which the work rate was increased by 30 W every three minutes. Ventilation (V E ), gas exchange measures, and blood samples for lactate analysis were obtained every third minute during the test. The onset of blood lactate accumulation was defined as the VO 2 at which there was a systematic rise in blood lactate concentration. Subjects were tested twice, once in a normal state of hydration (N) and once after acute thermal dehydration (D). The treatment order was counterbalanced. Thermal dehydration (5% body weight loss) was induced by intermittent sauna exposure (65° C). The onset of blood lactate accumulation occurred at a significantly lower (P < 0.05) VO 2 during exercise in the D condition when compared to work in the N condition. Additionally blood lactate concentrations were significantly greater (P <0.05) in condition D when contrasted to condition N at work rates of 90, 120 and 150 W, and at all work rates greater than 180 W. These findings demonstrate that acute dehydration may alter blood lactate accumulation and the onset of blood lactate accumulation.