The aim was to evaluate the difference of autonomic nervoussystem (ANS) activation before and after catheter ablation (CA)inpts with supraventricular tachycardia (ST) using the heart rhythmparameters. Methods: The ANS is as determinant of cardiac arrhythmia's.Heart rate variability was an accepted in 46 ST pts before and thenext day after successful CA. Frequency domain heart ratevariability was assessed by spectrum very low frequency (VLF)and low frequency (LF), which reflects modulation ofsympathetic and parasympathetic tone, high frequency (HF)described vagal tone and the ratio of LF/HF, which is used as anindex of sympathovagal balance. VLF also described humoralinfluence, LF+HF is used as reflex nervous influence. Results: RR intervals variability index LF/HF in lying positionbefore CA was 1,74 and after treatment - 1,32 (normal LF/HF>1,Pagani M. et al., 1986). Reflex nervous influence LF+HF to heartrhythm before CA was 39,0% and after CA - 34,4% (normalreflex nervous influence to heart rhythm65%>(LF+HF)55%, Zemaityte D., 1998). Conclusion: Decrease of sympathetic nervous system activationand reflex nervous influence of heart rhythm regulation was notedin patients with supraventricular tachycardia in early stage aftersuccessful catheter ablation.