These investigations are intended to develop methods by which sinoatrial and atrial arrhythmias may be evaluated and treated. Sinus node disorders result from inadequacy of the impulse generator (depressed automaticity) or a failure of conduction between a normally functioning impulse generator and the atrial myocardium (sinoatrial block). Utilizing cardiac stimulation techniques during cardiac catheterization we shall attempt to distinguish between these two abnormalities. The effects of vagal withdrawal and enhanced sympathetic tone will be explored. Programmed atrial stimulation is employed in the analysis of the mechanism of atrial arrhythmias. Currently, stimulation techniques employed in the analysis of atrial flutter suggest that this arrhythmia is initiated and sustained by intra-atrial reentry utilizing ordinary working atrial myocardium. His bundle tachycardias, atrial ectopic tachycardias, and ectopic atrial rhythms are similarly under investigation.