Blockade of the autonomic nervous system by injection of propranolol (0,2 mg/Kg) and atropine (0,04 mg/Kg) was carried out in a series of 48 patients classified in 3 groups:--Group I:sinus bradycardia (22 cases)--Group II:suspected tachycardia-bradycardia syndrome (14 cases)--Group III:suspected sinus node dysfunction (12 cases). Two parameters were studied after autonomic blockade: observed intrinsic sinus node frequency (OIF) and corrected "adjusted" sinus node recovery time (CASRT). The results were compared with those obtained during basal electrophysiological investigation. The OIF was abnormal in 26 cases (54%) and junctional rhythm was observed 9 times (19%). Prolongation of the post-stimulation pause occurred in 32 cases (67%), transformation from a normal to a pathological pause in 15 cases and normalisation of a pathological pause in 4 cases (8%). Sinus node recovery time did not change significantly in the other cases. There was a good correlation between OIF and CASRT when the OIF was abnormal and mediocre (48%), when the OIF was normal. The following conclusions were made:--CASRT improves diagnosis of organic sinus node dysfunction which may escape detection by basal electrophysiological investigation and excludes functional abnormalities.--OIF is of good diagnostic value when pathological. However, the integrity of sinus node function cannot be affirmed when this parameter is normal.