The recovery of post-exercise heart rate (HR) is enhanced by a procedure of cooling down; however, the mechanism of this facilitated reduction is unknown. To determine whether more cardiac vagal reactivation is associated with a decrease in HR following cool-down exercise, we examined high-frequency R-R interval variability (HF, 0.15-0.40 Hz), an index of cardiac vagal tone, in six young healthy male subjects with a comparison between sitting rest after 6 minutes of cool-down cycling at 20% peak oxygen uptake (VO2peak) and sitting complete rest, following 5 min of upright cycle exercise at 70% VO2peak. During the last minute of exercise, there was no difference in HR between the two exercise tests before performing or not performing cool-down exercise (mean +/- SE, 148.7 +/- 6.9 versus 149.7 +/- 7.0 beats/min, respectively, by a Wilcoxon signed-ranks test). After exercise, a similar initial rapid decrease in HR and subsequent decline was observed during the first 6 min of recovery, regardless of cool-down exercise. However, the resting HR average following cool-down exercise was significantly lower than the corresponding HR without cool-down exercise (92.1 +/- 3.0 versus 100.8 +/- 3.6 beats/min, P < 0.05). The corresponding HF amplitude was slightly, but significantly greater with than without cool-down exercise (10.6 +/- 2.2 versus 9.0 +/- 1.9, P < 0.05). Thus the increase in HF amplitude corresponded to a decrease in resting HR following cool-down exercise, as compared to complete rest without cool-down exercise. Therefore, we conclude that after moderate exercise, the decrease in resting HR following cool-down exercise is associated with an increase in cardiac vagal tone.