The purpose of this study is to evaluate the effects of a single intravenous infusion of K201 compared to placebo in a dose escalating manner on conversion to sinus rhythm, reduction of subject's symptom score, and safety.

Inclusion Criteria:

• Symptomatic atrial fibrillation for more than 3 hours and less than 7 days (as dated by symptoms).
• Atrial fibrillation documented by ECG at the start of study drug infusion.

Exclusion Criteria:

• Previous exposure to K201
• QTcF (Fridericia correction) >440 ms
• QRS interval > 140 ms
• Paced atrial or paced ventricular rhythm on ECG
• History of receiving another intravenous Class I or Class III antiarrhythmic drug within 3 days of randomization
• History of amiodarone (oral or IV) in the last 3 months.
• Clinical evidence or history of acute coronary syndrome (e.g. myocardial infarction, unstable angina) within 30 days prior to randomization
• History of failed electrical cardioversion at any time in the past
• History of polymorphic ventricular tachycardia (e.g. torsades des pointes)
• History or family history of Long QT Syndrome
• History of ventricular tachycardia requiring drug or device therapy
• Ejection fraction of 40% or less.