To assess the variability of the currently used electrophysiologic parameters and their possible circadian rhythm, sequential bedside electrophysiologic testing was performed during a 24-hour period, at intervals of 1 to 2 hours, in 12 patients who had normal atrioventricular (AV) conduction times and normal sinus node function. The coefficients of variation during the 24-hour period were: ± 10.4% for the R-R interval, ± 10.6% for the sinus node recovery time (SRT) at atrial pacing of 100 bpm, ± 32.5% for the corrected SRT, ± 15.1% for the ventriculoatrial (VA) effective refractory period (ERP), ± 8.3% for the AV nodal ERP, ± 5.7% for the AH interval, ± 5.2% for the HV interval, ± 5.5% for the atrial ERP, ± 3.3% for the right ventricular ERP, ± 2.8% for the QT interval, ± 4% for the VA interval, and ± 3.4% for the retrograde Kent bundle ERP. Between 12:00 midnight and 7:00 am, there was significant lengthening of: the sinus node rate ( p < 0.0005), the SRT at atrial paced rates of 100 and 120 bpm ( p < 0.025), the QT interval duration ( p < 0.025), and the ERP of the atria ( p < 0.025), AV node ( p < 0.01), and right ventricle ( p < 0.05). Thus conventional electrophysiologic parameters are subject to daily variability and, like sinus node function, AV nodal and myocardial refractoriness follow a circadian rhythm with an acrophase between 12:00 midnight and 7:00 am. In addition, prolonged bedside recording of the His bundle potential can be reliably obtained.