A ten-year-old Warmblood gelding presented to the Equine/Farm Animal Hospital at Cornell University with a two-week history of atrial fibrillation. Cardiac auscultation revealed an irregularly irregular rhythm and a grade II/VI left-sided systolic murmur. An ECG confirmed atrial fibrillation and an echocardiogram revealed no structural abnormalities. A mild amount of mitral regurgitation was found on Doppler. Traditional treatment for atrial fibrillation is with quinidine. While this treatment is about 85% effective, side effects range from mild to severe. Other experimental drugs have been used, including flecainide and amiodarone. A promising new treatment is electrical cardioversion. Under general anesthesia, electrical shocks are delivered via two electrical catheters placed in the left pulmonary artery and right atrium. The horse in this case successfully returned to sinus rhythm after the third shock. He has a good prognosis although there is a chance he could have a recurrence of atrial fibrillation.