In a studied group of 1727 subjects in 101 cases (of either sex) the sick sinus syndrome with MAS attacks++ was diagnosed on the basis of clinical and electrocardiographic criteria. The most frequent cause of the syndrome with paroxysmal atrial fibrillation was ischaemic heart disease. In 42 cases atrial pacing was applied measuring the time of own rhythm return after the end of pacing, and His bundle potentials were recorded. For practical reasons the patients were divided in two groups. Trials of treatment with atropine or isopropylnoradrenaline derivatives++ were undertaken in group A, and in group B antiarrhythmic++ drugs were used (beta-blockers, quinidine, isoptin etc). In group I (38 patients with striking bradycardia) a good therapeutic effect was observed after pharmacological treatment in 8 cases (21%). In group II (63 cases of the tachy-bradycardia syndrome) a good effect was observed in 11 cases (17.4%).