The aim of the study was to analyze the effect of surgical moderation of conduction from the atrium to the atrio-ventricular (AV) node on ventricular response in 30 patients with atrial fibrillation (AF) undergoing mitral valve replacement (MVR). Besides the usual diagnostics, echocardiographic evaluation of the size of the left atrium was performed in all patients, as a parameter of stability of rhythm conversion by drugs. Surgical treatment was applied exclusively in cases where, judging by these parameters, there was little chance for drug treatment to be effective. All patients (mean age 50 years) had severe mitral valve disease (NYHA III and IV), chronic AF and irregular ventricular rate, and underwent mitral valve replacement with mechanical valve. The patients were divided into three groups of ten patients each. In group I: MVR was combined with interatrial septal incision (IASI) to study the effect of the moderation of conduction from the interatrial septum to the AV node; in group II: after MVR, IASI was combined with crista terminalis myotomy (CTM) to study the additional effect of the moderation of conduction along the crista terminalis to the AV node; in group III (control): MVR was performed through left atriotomy, without antiarrhythmic incisions. Ventricular response was studied in two phases: In the immediate postoperative period (up to 7 days), using continuous electrocardiographic (ECG) monitoring, and 2 months postoperatively, using 24 h Holter monitoring with the results compared to a preoperative 24 h recording. Postoperative AF occurred in all patients.(ABSTRACT TRUNCATED AT 250 WORDS)