Under investigation in this study were the relationship between hypersensitive carotid sinus reflex (HCSR) and sick sinus snydrome (SSS), the significance of clinical symptomatology versus electrophysiologic test results and the natural course of both syndromes. In 186 symptomatic patients (pts) resting heart rate, maximal corrected sinus node recovery time (CSRTmax) and maximal carotid sinus pressure results (CSPmax) were determined. According to test results, 103 pts had HCSR (I), 33 had HCSR + SSS (II), 30 pts had SSS (III), and 20 pts with normal test results served as controls (IV). Mean age was the same in groups I--IV (p greater than 0.05). Groups I--III had a heart rate less than or equal to 60 bpm. There was no correlation between patients test data and the occurrence of syncopes in each of groups I--III (p greater than 0.05). In 16 pts with SSS, test results remained unchanged 16 months later (p greater than 0.05). More pts (31%) in group II died in a shorter period of time after pacer application (1.4 +/- 1 year) than pts with AV III degrees block (25%, 1.8 +/- 1.6 years) after pacer application. We conclude that the combination HCSR + SSS seems rather frequent. Specific testing separated HCSR from SSS, but failed to predict syncopes and thereby cannot aid in the indication for pacer application. The 16 months prognosis of SSS proved unfavorable. Mortality after pacer application in HCSR + SSS seems less favorable than in AV III degrees block (p less than 0.05), but depends largely on the severity of associated diseases.