A permanent supraventricular tachycardia (SVT) was diagnosed in a 54-year-old hypertensive but cardiologically asymptomatic female patient, admitted to a surgery department for biliary lithiasis and hepatic echinococcosis. Heart rate was about 130 bpm and ECGs showed negative P waves in leads I, II, III, aVF, and precordial leads V2 to V6, being the RP' interval longer than P'R interval. Pharmacological intervention during Holter monitoring (20 hours) was instituted: following i.v. propranolol (4 mg), heart rate progressively decreased (to 112 bpm), mainly due to an increase in SVT RP' interval, and brief, spontaneous SVT interruptions occurred, preceded by P'R interval prolongation; SVT stopped after P' recording, and resumed after 2 sinus beats, (showing enlarged P waves and slightly prolonged PR interval), induced by cycle length shortening; later on, under i.v. amiodarone infusion (100 mg/hour) and coincident with the sleeping period, SVT cycle length progressively increased (to 600 msec), due to equivalent increases in P'R and R'P intervals. Two premature ventricular contractions (PVC) occurred during Holter monitoring at a coupling interval of 80-85% of SVT cycle length (480 msec): one PVC apparently originated in left ventricle lateral wall, captured the atria, which were activated 75 msec earlier than expected; the other PVC, apparently originated in left ventricle septoapical region, did not interfere with SVT cycle length. Before these data, a diagnosis of circus movement tachycardia, incorporating a concealed accessory pathway with slow retrograde conduction and ventricular insertion in the postoroseptal or left posterior paraseptal region, and showing minor impairment of antegrade AV nodal conduction, was made. Invasive electrophysiological study was then discarded. With combined oral antiarrhythmic therapy (amiodarone, 600 mg/d), plus propafenone, 450 mg/d), sinus rhythm was permanently restored, with evidence of intraatrial block, slightly prolonged PR interval and no preexcitation. Transesophageal echocardiography revealed a small atrial septal aneurysm associated with a small atrial septal defect; echocardiographic features were consistent with the hypothesis of incomplete regression of the atrial septal aneurysm after partial closure of the atrial septal defect. Abdominal surgery (cholecystectomy plus partial hepatic pericystectomy) was performed without any complications or SVT recurrences. During a 6-month follow-up period, maintaining amiodarone (200 mg/d) and propafenone (450 mg/d), the patient remained SVT-free, and Holter monitoring performed at 3 and 5 months showed permanent sinus rhythm and 1:1 AV conduction with slightly prolonged PR interval (less than 0.29 sec and shortening at faster heart rates). This case documents Holter monitoring capability for the evaluation of tachycardia mechanisms in patients with permanent SVT.