Thirty-three patients with disorders of sinus node function treated with permanent endocardial pacemakers were evaluated. Study of the underlying heart rate and rhythm was accomplished by external inhibition of pacemakers. The development of stable atrial fibrillation was documented in 7 out of 29 patients studied and effectively terminated the syndrome of sinus node dysfunction. Embolic complications appeared to be an important factor in the morbidity and mortality in patients with changing supraventricular rhythms. Pacemaker therapy effectively controlled syncopal episodes due to bradycardia, but recurrent episodes of tachycardia and problems associated with this remained unaffected.