Background Inappropriate sinus tachycardia IST manifests chronic nonparoxysmal sinus tachycardia in healthy individuals and is characterized by consistently elevated heart rate and exaggerated responses to minimal physical activity. Heart rate variability HRV using 24-hour Holter monitoring represents one of the methods of evaluating the harmony of autonomic nervous system activity. So, authors investigate the autonomic nervous system activity by the HRV in patients with IST. Methods We compared the pattern of cardiac sympathetic and parasympathetic activity through the time domain analysis of heart rate variability with 24-hour Holter monitoring between 23 patients with IST and 23 healthy control subjects. None of the patients had organic heart disease as determined by routine laboratory examination and echocardiography. And then, time domain measures mean NN, SDNN, SDANN, SD, rMSSD, pNN50 were analyzed. Results The mean age of patients and control were 38 10 and 37 9 years, respectively p NS . The mean clinical heart rate of patients and control was 104 5/min and 72 5/min, respectively p 0.05 . Among the time domain indices, mean of all normal RR intervals mean NN , standard deviation of all normal RR intervals SDNN , standard deviation of mean RR interval SDANN , mean of standard deviations of all normal RR intervals SD , root mean square successive differences between adjacent normal RR intervals rMSSD , and percent of difference between adjacent normal RR intervals pNN50 in the patient group were significantly shorter compared to control group p 0.01 . Conclusion In IST, cardiac vagal influence on the heart rate is blunted. Korean Circulation J 2000 ; 30 9 : 1133-1138