The tests recommended for establishing the diagnosis Sick Sinus Syndrome (SSS) were carried out on 36 patients with non-drug-induced Sinusbradycardia (16 symptomatic, 20 asymptomatic) and their resp. diagnostic value was then analysed. Contrary to previous publications, response of heart frequency to exercise testing was normal in all subjects as well as response to intravenous Atropine administration with the exception of 3 cases. A good correlation was found between typical arrhythmias in long-term Ecg monitoring and clinical symptoms. A pathologically prolonged Sinus Node Recovery Time (SNRT) is found in the same number of symptomatic and asymptomatic subjects, frequency-corrected SNRT shows the same results. Shortening of SNRT after Atropine is the more pronounced, the longer SNRT was before Atropine administration. Non-normalization and paradoxical prolongation of SNRT after Atropine have also been observed. In 8 patients stepwise progressive atrial pacing beginning with pacing-frequencies of 70 p.m. was performed before and after Atropine administration. Observation of the effect of Atropine on SNRT may prove to be more important for prognosis and treatment of the SSS than the finding of a prolonged SNRT for establishing the diagnosis. The diagnosis "SSS" should not be based on a pathologically prolonged SNRT alone. Analysis of long-term Ecg monitoring and clinical symptoms should be the main diagnostic tool.