This study examined how the relationship between heart rate (HR) and relative oxygen intake (% VO_2 max) changes with exercise time in continuous running, in as much as HR and % VO_2 max are the most frequently used indices of work intensity of aerobic exercise. Subjects were four male middle and long distance runners and one untrained male. Experimental exercise was a treadmill running without inclination at constant velocity. Running velocities used were three to five levels at which the subjects exhausted in four to thirty minutes, and additional three to five levels at which the subjects were able to run more than thirty minutes. The exercise was conducted under the temperature condition of 20-30℃. The results are summarized as follows: 1. For the purpose of this study, in the first step, two equations estimating HR and oxygen intake (VO_2) were derived from running velocity (V) and exercise time (t), variations of these due to the experimental sessions were known to be very small. HR or VO_2 estimated from these equations well approximated the values measured. These two equations were respectively denoted (6) and (7) in the course of discussion: HR= a(t)+b(t)・V+c (t)・V^2-(6), VO_2=A(t)+B(t)・V+C(t)・V^2-(7),Where the coefficients in the equations were quadratic functions of exercise time t. In the next step, eliminating V in the equations (6) and (7) VO_2 was estimated from HR and exercise time at which the HR appeared Relative error of estimate of all data measured was 4.06±0.63% (3.28-4.94%) for the average of five subjects. Since the values thus estimated well approximated the values actually measured, the procedure estimating VO_2 from HR and exercise time may be accepted valid. 2. Based on the relationship between HR and % VO_2 max in function of exercise time, it was found that % VO_2 max at a given relative heart rate (% HR max) became smaller with the increase in exercise time. It was also found that the higher the % HR max, the larger the rate of the decrease was, and that % VO_2 max at the thirty minutes in the case of 100 % HR max (l85.2±7.5 beats.min^-1) was about 24 % sma1ler than that for the same case at the four minutes. These results suggest that % VO_2 max might be overestimated for the longer exercise time even if % HR max wax the same, and that the higher the % HR max, the larger the degree of overestimation would be. 3. It may be concluded that exercise time at which the HR appeared should be taken into account when using HR as an index of work intensity especially under the condition of high temperature (about 20-30℃).