OBJECTIVE: To discuss the clinical significance of heart rate turbulence (HRT) after atrial premature beats in accessing the temporal changes of vagus nerve before the onset of paroxysmal atrial fibrillation. METHODS: The HRT parameters: turbulence onset (TOA) and turbulence slope (TSA) after atrial premature impulses occurring 0 to 60 min before and after the onset of AF 56 patients with atrial fibrillation (AF). The heart rate variability (HRV) time domain parameter SDNN and the frequency-domain parameters, low frequency (LF), high frequency (HF), and LF/HF ratio were also analyzed. TOA and TSA were calculated for 3 periods reflecting morning hours (6-10 am), afternoon hours (1-5 pm), and night hours (0-4 am). RESULTS: (1) The TOA and TSA were significantly higher 1 h preceding AF than 1 h after AF(0.47+/-0.32 vs 0.24+/-0.18 and 16+/-6 vs 13+/-4, both P<0.05). The HRV time domain parameters and frequency-domain parameters did not change remarkably after the onset of PAF. (2) The TOA and TSA were significantly higher in night hours than in morning hours (1.2+/-0.6 vs 0.8+/-0.4 and 17+/-6 vs 14+/-4 respectively, both P<0.05), the two parameters of HRT maintained significant circadian variations. CONCLUSION: Vagal inhibition in response to premature atrial excitation is absent and transient enhancement of vagal outflow would occur before AF. HRT before atrial premature beats is of significance in accessing the changes of autonomic nervous system before onset of AF.