Objective:To observe the characteristics of 24 hour ambulatory electrocardiogram in patients with chronic atrial fibrillation (AF) and optimal ventricular rate control. 　　Methods:Holter monitoring was performed in 122 hospitalized patients,aged 17—85 (mean 51.57±11.94) years,with organic heart disease and chronic AF (New York Heart Association functional class Ⅱ-Ⅲ) in whom the ventricular rate were well-controlled by digoxin.Maximum,minimum and mean ventricular rates,the number of ventricular pauses of ＞1.5 second,and the longest ventricular pauses during the monitoring were recorded.Follow-up data were collected in 72 patients. 　　Results:The mean,fastest and lowest ventricular rates over 24 hours were 78.53±8.12(65—90)，143.79±28.85(90-183) and 51.04±7.52(34—71) bpm,respectively.Bradycardia of less than 50 and 45 bpm occurred in 62 and 32 of the 122 patients,respectively.Ventricular pauses of more than 1.5 and 3.0 seconds were noted in 95 and 14 of the 122 patients,respectively.The longest ventricular pause was 2.38±0.55(4.64—1.60) seconds.During follow-up,there were no worsening in these patients. 　　Conclusion:Ventricular pauses at rest and nocturnal bradycardia were common in patients with chronic atrial fibrillation who had ventricular rates controlled.The dosage of digoxin should not be changed,nor pacemaker insertion is needed.