We studied the effects of prajmaliumbitartrate (PBT, Neo-Gilurytmal, Giulini Pharma, Hannover, FRG), an antiarrhythmic drug on some parameters of blood coagulation in patients. PBT, 20 mg given three times a day, significantly prolonged template bleeding time and ex vivo thrombus formation time in a modified Chandler's loop. Ex vivo platelet aggregation using different agonists and tests of the plasmatic coagulation system remained unchanged.