Cardiovascular responses during 30 min of exercise in 20°C, 30°C, and 40°C(R.H.50%) were measured in six male Japanese. The subjects exercised on a bicycle ergometer at the two kinds of work loads, i.e., 300 and 600kgm/min. Cardiac output(Q) was detennined with the CO₂ rebreathing method at the 5th, 15th, and 30th min of exercise. Oxygen uptake(V_O_2), heart rate(HR), rectal temperature(Tr), and mean skin temperature(Ts) were also measured. V_O_2 and Q did not change siguificantly during exercise in all environments. Analysis of variance showed that the factor of ambient temperature had no siguificant effects on V_O_2 and Q. HR increased with time and SV decreased with time during exercise. The largest change in HR and SV occured between the 5th min and the 15th min of exercise. Tr increased with time during exercise. The marked increase in Tr occured between the 15th min and 30th min of exercise. The regression equations of HR on Q and SV on Q in each time of exercise were calculated. The analysis of covariance showed that the elevation of regression line of HR on Q in the 5th min of exercise was significantly lower than those in the 15th min and the 30th min of exercise. Similarly, the elevation of regression line of SV on Q in the 5th min of exercise was significantly higher than those in the 15th min and the 30th min of exercise. From the regression equation of SV on Tr the adjusted values of SV for each time of exercise were calculated in order to exclude differences in Tr. The adjusted values of SV in the 5th min of exercise was siguificantly higher than those in the 15th min and the 30th min of exercise. So the alterations in HR and SV observed between the 5th min and the 15th min of exercise may accounted for by other than the change in internal body temperature.