Background There is evidence that the autonomic nervous system may be involved in the mechanism of focal atrial fibrillation (AF), so the present study investigated the effects of the parasympathetic nervous system on the occurrence of focal AF originating from the pulmonary veins (PVs). Methods and Results In 10 mongrel dogs, programmed stimulation and local burst stimulation (12.5 Hz, impulse duration 0.5 ms) were performed at each of the PVs. Pacing thresholds at different sites were determined and shown as a terraced distribution. The closer to the ostium of the PV, the lower was the pacing threshold (P<0.05-0.001). The local effective refractory period (ERP), AF induction and AF threshold were measured at baseline and during bilateral vagal nerve stimulation (VNS). VNS led to local ERP shortening at each of the PV sites (P<0.05-0.001), increased the inducibility of AF at all sites in the 4 PVs (P<0.05-0.001), and decreased the AF threshold at most sites, especially in the distal portions of the 4 PVs (P<0.05-0.01). Conclusions VNS changes the electrophysiological characteristics of the PVs and facilitates the induction of AF. Interaction between the autonomic nervous system and local cardiac autonomic nerve system may be a potential mechanism. (Circ J 2009; 73: 48 - 54)