The heart rates of 14 voluntary subjects (seven male and seven females) participating in a 5 d jogging relay were recorded. The jogging speed was controlled at 3.0 m.s-1 on average. The heart rate measurements were related to cardiorespiratory responses obtained during VO2max test on a treadmill. The mean (s.d.) VO2max values were 53(2) and 48(2) ml.kg-1.min-1 and the anaerobic threshold was 42(1) and 38(2) ml.kg-1.min-1 for male and female subjects respectively. During the relay the mean(s.d) heart rate values of male and female subjects were 150(1), and 167(3) beats.min-1 (P < 0.001), corresponding to 68(1) and 76(2)% VO2max (P < 0.01) as calculated from the individual HR/VO2 regression equations. The jogging time above anaerobic threshold heart rate level was 9(3)% for the males and 43(7)% for the females (P < 0.001) of the total jogging time. The results indicate that even in a leisure oriented jogging event, cardiorespiratory strain can be rather high and that the strain can be significantly higher for female than for male subjects. The determination of the anaerobic threshold as well as VO2max is important when quantifying the level of cardiorespiratory strain of exercise.