The purpose of the present study was to examine the relationship between blood lactate (La) and blood osmolarity (Osm) during high intensity constant load exercise in heat. Seven male subjects (age; 20.7±1.4yrs, height; 174.1±4.9cm, body mass; 68.1±4.3kg, VO_2max; 47.3±5.8 ml・kg^<-1>・min^<-1>) performed a 80%VO_2max exercise for 10 min by using cycle ergometer on two different conditions : an ambient temperature of 20℃ and 50% relative humidity (control; CON) and 40℃ and 50% relative humidity (heat trial; HT). Trials were conducted within a week in random order. Forearm skin blood flow during exercise were higher in HT compared with CON (P<0.05), however, no difference in oxygen consumption during exercise was observed between HT and CON. These results suggest that blood flow to active muscles during exercise may be unchanged by HT. In spite of the similar oxygen consumption, exercise in HT resulted in a higher La at 3 min during exercise compared with CON (6.0±0.3mmol・1^<-1> vs 5.0±0.6mmol・1^<-1>, P<0.05). In contrast, Osm was unchanged in HT during exercise and recovery compared with CON. In addition, the changes in Osm were highly correlated with the changes in total blood electrolytes (Na^+, K^+ and Cl^-) including La^- (r=0.98, P<0.05). It is suggested from these results that the changes in Osm during exercise in HT and CON may be controlled by the change in total blood electrolytes but not only the change in La^-. The reason for elevated La in HT at earlier time during exercise was unknown.
