Abstract Studies on the relationship between autonomic nervous system and the onset of paroxysmal atrial fibrillation in humans have yielded conflicting results. The purpose of this study was to assess the difference of alteration of autonomic tone according to the pattern of heart rate change immediately before the onset of atrial fibrillation by spectral power analysis. From a database of Holter recordings taken from patients with symptomatic PAF, we selected 43 episodes of AF, lasting more than 30 s, initiated by a single ectopic beat and preceded by sinus rhythm for more than 1 hour from 31 patients(21 men & 10 women aged 58짹12 years). The 20 minutes before PAF divided into 10 2-minutes periods and were studied using repeated measures ANOVA. According to the change in heart rate over the final period of sinus rhythm, 43 episodes were classified as group A(n=23), in which acceleration of heart rate was observed before PAF, and group B, in which heart rate acceleration was not observed before PAF(n=20). In case of AF episodes belong to group A, AF were preceded by a significant decrease in HF components(HFnu, lnHF ; normalized values and natural logarithm-transformed values)(p<0.0001 and p=0.005, respectively), rather than increase of LF component before PAF onset. LF/HF ratio showed a significant linear increase before onset(p=0.019). In contrary to group A, a significant increase in HF components(HFnu, lnHF)(p<0.05 and p=0.015, respectively) and a decrease in LF/HF ratio were observed in group B AF episodes. No significant changes were observed in LF components(LF, LFnu, lnLF) in both types of AF. In conclusion, the occurrence of PAF depends on variations of the autonomic tone and parasympathetic modulation has a key role in modulation and initiation of AF. Analysis of autonomic tone change should be applied differently depending on the types of AF.