The aim of this study was the electrophysiological evaluation of bisoprolol, a new highly cardioselective beta blocker, void of intrinsic sympathomimetic activity and without significant membrane stabilizing action. 10 patients (4 males and 6 females, mean age 67.5 years) with cardiac arrhythmias or syncopes of undetermined origin underwent His bundle recording with incremental as well as programmed atrial and ventricular pacing in an electrophysiological study. Electrophysiological parameters were measured at basal conditions and at 15 and 30 minutes after the infusion of bisoprolol 5 mg (for patient 1 to 5) and 10 mg (for patient 6 to 10). All electrophysiological parameters were within normal range at basal evaluation. Bisoprolol prolongs significantly sinus cycle length, corrected sinus node recovery time, AH interval, cycle length inducing AV-node block, effective and functional refractory periods of the AV-node. All these parameters remained within normal limits and were modified to about the same extent by 5 and 10 mg of the drug. No drug-related changes of QT interval were observed. Bisoprolol have been confirmed to have strictly beta-blocking properties.