Interruption of atrial flutter and fibrillation by RF catheter ablation may be favored by large, elongated lesions. We administered RF current in unipolar and bipolar mode in porcine right atrium. Bipolar ablation was performed between the tip electrodes of two serially coupled catheters. With 4-mm tip electrodes in vitro, lesion length increased from a mean (SD) of 7.9 (1.2) mm at 3 mm-interelectrode distance (i.e.d.) to 13.3 (3.3) mm at 9-mm IED, but decreased at 12-mm IED due to nonconfluent lesions (P < 0.0001). With 4 mm distal electrodes and 8 mm IED, bipolar lesions were 65% longer than corresponding unipolar ablations. Switching to bipolar mode increased the lesion length more than increasing electrode tip length to 6 mm in unipolar mode. Power and temperature controlled ablation created equally sized lesions. Twelve anesthetized pigs were randomized to unipolar or two catheter bipolar temperature controlled ablation of the right atrial free wall. Bipolar ablation created confluent lesions with endocardial length x width of 13.5 (5.8) x 7.3 (3.7) mm, unipolar ablation 6.4 (2.8) x 4.6 (1.4) mm (P < 0.001 when comparing length and P = 0.013 for lesion width). The atrial lesions in both groups were transmural and extended into hilar lung lesions with maximal depth of 3.0 (1.1) and 2.6 (1.0) mm, respectively (P = 0.44). Five bipolarly and four unipolarly ablated pigs developed right diaphragmal paresis. We conclude that bipolar ablation may be preferable in situations where large, elongated lesions are favorable. The two catheter technique is feasible in porcine right atrium. Both bipolar and unipolar ablation of the porcine right atrial free wall may frequently be complicated by injury to the phrenic nerve and adjacent lung tissue.