We investigated a novel device that delivered biphasic and monophasic shocks without tilt, in transvenous cardioversion of atrial fibrillation (AF). The device contains a radiofrequency (RF) transmitting primary, a 15mm air gap and a passive secondary unit that produces the required direct current waveform. Methods: The device was assessed in 10 anaesthetised and ventilated sheep (65 ± SD 12kg). Defibrillation leads were positioned in the distal coronary sinus and the lateral right atrium. The novel RF defibrillator was assessed using rectangular monophasic or biphasic pulses and compared to conventional (C) tilted waveforms (Ventritex HVS-02). During AF placebo shocks were delivered to a 50 Ω dummy load. Sustained AF was induced by rapid atrial pacing and 5 attempts at cardioversion were made for each waveform. Success was defined as reversion to sinus rhythm within 5 beats of shock delivery. Results: No arrhythmic complications were observed for the 400 shocks delivered. Conclusion: A reduction in phase 2 voltage does not affect the efficacy of rectangular waveforms but does facilitate energy reduction. Asymmetric rectangular biphasic waveforms with phase 2 voltage at 50% of phase 1 are more efficacious than equivalent voltage conventional waveforms for transvenous cardioversion of AF. The novel defibrillator is safe and effective. Design modifications will enable this device to be used as an implantable defibrillator with an external RF power source.