Dronedarone-d is intended for use as an internal standard for the quantification of dronedarone by GC- or LC-MS. Dronedarone is an antiarrhythmic agent and a derivative of amiodarone . It inhibits muscarinic acetylcholine receptor-dependent potassium currents induced by carbachol (carbamoylcholine;) or intracellular loading of GTPγS in single cells isolated from guinea pig atria (ICs = 0.1 and 1 µM, respectively). Dronedarone (50 and 100 mg/kg) reduces sinus frequency and prolongs action potential duration (APD) in isolated rabbit ventricular myocardium. It increases atrial APD and the atrial effective refractory period (AERP) in a dog model of atrial fibrillation induced by rapid right atrial pacing. Formulations containing dronedarone have been used in the treatment of atrial fibrillation.