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The difference in sympathovagal activity preceding non-sustained ventricular tachycardia(NSVT)was examined between patients with and without a circadian rhythm. Thirty-three patients' Holter monitoring data(4INSVT episodes)were analyzed regarding the frequency domain measures(low-frequency component[LF : 0.04-0.15Hz], high-frequency component[HF : 0.15-0.4Hz], and the ratio of LF to HF[LF/HF])for each 15-min average from 120 min before each episode of NSVT. The presence of a circadian rhythm was accepted when the rhythm adaptation was significant by cosinor analysis and the acrophase was located at night(22.00-06.00h)in HF(HF-positive group, n=17), and during the daytime(10.00-20.00h)in LF/HF(LF/HF-positive group, n=12). The negative groups were identified by the absence of a circadian rhythm(HF-negative group, n=16 ; LF/HF-negative group, n=21). The serial changes in the HF power before NSVT were significantly different between the HF-positive and -negative groups(p<0.05). The HF increased from 75-60 min before NSVT in the HF-positive group, whereas the HF decreased from 60-45 min in the HF-negative group. The serial changes in the LF/HF ratio were not significantly different between the LF/HF-positive and -negative groups. Thus, the circadian rhythmicity of vagal activity seems to have an important role in the genesis of NSVT.