A comparative investigation of the electrophysiological properties of the sino-atrial node, the right atrium, the AV-node, and the intraventricular conduction system was performed in 37 patients. 18 patients had a sino-atrial block, 8 a persistent bradycardia and 11 a brady-tachy syndrome. There are the following conclusions: 1. The greatest dysfunction of the sino-atrial node was observed in the group of patients with sino-atrial block. This can be derived from the determination of the sinus node recovery time and from the occurrence of secondary pauses. 2. The vulnerability of the right atrium, investigated by the extra-stimulus-technique, is enhanced in the brady-tachy syndrome. Measuring the duration of the P-wave and the PA-interval does not give any information about the atrial dysfunction. 3. Patients with a sino-atrial block often have a disturbed AV-node conduction. In most patients it is latent only and easily can be detected by atrial incremental pacing. 4. A hemiblock or bundle branch block is found more often in patients with sino-atrial block and sinus bradycardia than in patients with the brady-tachy syndrome. By the registration of a His-bundle-electrogram a trifascicular block can be uncovered in half of these patients. 5. In the sick sinus syndrome, there is always the possibility of additional dysfunctions in the conduction distal to the sino-atrial node. With special regard to therapeutic measures, a complete evaluation can only be made by of both endocardial recording and stimulation methods.