Atrial fibrillation is a typical disturbance of heart rate in patients with mitral valvular disease. Correction of the mitral valvular disease by operation of replacement (or plasty) of the mitral valve normalizes the intracardial hemodynamics resulting in clinical improvement of the patient's state and prognosis for the nearest and long-term period. Atrial fibrillation aggravates the course of the heart disease, is a substantial factor associated with thromboembolic complications.The heart rate dynamics after mitral valve replacement was assessed in 94 patients. The sinus rhythm in patients after mitral valve replacement is not changes, if there were no episodes of atrial fibrillation before the operation. The presence of chronic atrial fibrillation before operation suggests this arrhythmia to be present after correction of the mitral valvular disease if no additional operation correcting the rhythm was made. Operation of atrial fragmentation by the "labyrinth" type allows the simus rhythm to be restored in most patients with chronic form of atrial fibrillation and in long-term periods--in half of the patients.