20 patients with atrial fibrillation and rapid ventricular response were treated with the new calcium-antagonist Gallopamil. The effects on heart rate were evaluated by Holter-monitoring. A significant decrease of heart rate with a duration of 10 hours was found in 10 patients after acute oral administration of 100 mg Gallopamil. In 5 patients a regularization of the ventricular response was observed, suggesting an AV junctional escape rhythm during AV nodal block. However, no relevant bradycardia was seen. The maximal decrease of heart rate was seen 2 to 4 hours after application. Oral administration of 3 times 50 mg Gallopamil daily decreased heart rate after a treatment period of 1 week to about 79% of its control values. This effect started 1 hour after application of the first dose in the morning and it lasted for 7 hours after the evening dose.