The purpose of this study was to compare blood lactate concentration, lactate threshold (LT) and onset of blood lactate accumulation (OBLA) during an incremental bicycle exercise under a hot dry environment with those during the same workload under a thermoneutral environment. Eight unacclimated men performed an incremental test to exhaustion on a cycle ergometer during which the work rate was increased by 30 W every three minutes under thermoneutral (25°C) and hot dry (40°C) environmental conditions. Oxygen consumption (VO₂) -pulmonary ventilation (VE), gas exchange measures and earlobe blood samples for lactate analysis were obtained every minute during the test. LT (p<0.05) and OBLA (p<0.01) occurred at significantly lower VO₂ under the hot environment than those under the thermoneutral environment. Additionally blood lactate concentration was significantly higher (p<0.05) at each measurement period under the hot environment compared with that under the ther-moneutral environment. The correlation between LT and ventilatory threshold (VT) was not statistically significant under the thermoneutral (r=0.20) and hot dry (r=0.60) environments, These findings demonstrate that the hot dry environment may increase blood lactate concentration more and causes a leftward shift of LT and OBLA. Since blood lactate accumulation during incremental exercise is not considered to be the only factor which mediates VE, VT does not always accurately predict LT.