Double blind study to assess versus placebo the efficacy of SSR149744C for the conversion of atrial fibrillation/flutter to sinus rhythm at the time of the planned electrical cardioversion.

This is a dose ranging multicenter, multinational, randomized, double blind, placebo controlled, parallel arm study. Patients with AF/AFL will be randomized to one of two doses of SSR149744C.

The efficacy of SSR149744C will be based on the proportion of patients converting to normal sinus rhythm at the end of the treatment period (48 hours post first dose).

Patients will receive the study drug on D1 and D2. Patients not converting pharmacologically to sinus rhythm will undergo an electrical cardioversion on D3.

Inclusion Criteria:

• AF/AFL for >72 hours and documented by 2 12-lead ECGs separated by at least 72 hours and indication for electrical cardioversion of the current AF/AFL episode as judged by the investigator.

Exclusion Criteria:

MAIN CRITERIA (non-exhaustive list):

• Women of childbearing potential without adequate birthcontrol, Pregnant Women, Breastfeeding women, conditions which increase the risk of severe antiarrhythmic drug side effects, permanent pacemaker, permanent AF/AFL, contraindication to anticoagulant, severe left ventricular dysfunction, severe associated conditions.