Purpose: The purposes of this study were to compare the magnitude and phase between the RR interval and QT interval variability in the frequency domain. METHODS: Twenty four, 12-13 year old healthy males were randomly selected. At resting state and for 5 minutes, ECGs were obtained, and they were digitized to 1000Hz. After measurement of RR interval, QT interval variability was measured using template matching strategy. After normalization of the RR and QT interval time series, power spectral and cross spectral analysis were performed. From each of the time series, low- (0.04-0.15 hertz) and high- (0.15-0.4 hertz) frequency power were measured. From the phase spectrum, the phases and time lags between the two time series at each of the two frequency range were calculated. RESULTS: The average of RR interval and QT interval was 616.0+/-71.0, 364.0+/-47.0 msec, respectively. Their normalized low- and high- frequency power was 4.4+/-7.9 NU(normalized unit), 0.1+/-0.1 NU(p<0.005), and 11.0+/-30.0 NU, 0.3+/-0.3(NU, p<0.005), respectively. The phase differences and resulting time lags between the two interval were -0.5+/-0.4 pi radian(-0.9 seconds) and -0.2+/-0.3 pi radian(-0.4 seconds) in the low- and high-frequency range, respectively. CONCLUSION: During resting state, when compared to RR interval, QT interval oscillates in significantly lower amplitude in both low- and high- frequency ranges. However, the oscillations precede those of the RR interval 0.9 seconds and 0.4 seconds, respectively.