The purpose of this study was to investigate the recovery kinetics of cardiac autonomic regulation after acute exercises related to exercise intensity. Firstly, eight healthy subjects performed two kinds of constant load exercises at the work rate corresponding to 20% and 100% of the individual ventilatory threshold (VT) in addition to the exhaustive incremental exercise using a cycle ergometer. Blood pressure (BP) and oxygen uptake (VO2) were measured during 9 to 10 min after the exercises as well as the beat-by-beat recording of R-R intervals. Coarse graining spectral analysis (CGSA) was applied to heart rate variability (HRV) data sets of 5 min before exercise, last 5 min during exercise, and 8 to 10 min after exercise. The low frequency (0-0.15 Hz; LO) and the high frequency (0.15-0.5 Hz: HI) areas under power spectra were calculated for evaluating sympathetic (LO/HI) and vagal (HI) activities. The recovery for 10 min was sufficient to settle both VO2 and BP even after the exhaustion. Comparing to the pre-exercise value, however, HI was still suppressed until 10 min after the 100% VT exercise (522 +/- 300 vs. 122 +/- 63 msec2, p less than 0.05) while it was recovered at 10 min after the 20% VT one (353 +/- 122 vs. 487 +/- 159msec2). Secondly, six healthy subjects performed an incremental cycle exercise until exhaustion. The R-R intervals were monitored beat-by-beat during 30 min after exercise. The CGSA was applied to every five min data set of HRV during recovery phase.(ABSTRACT TRUNCATED AT 250 WORDS)