The supernormal phase of excitability of the human heart was studied by means of fixed rate endocardial pacing in 11 patients with acute and chronic bradyarrhythmias. Ten of the eleven patients manifested a supernormal phase. The duration of the phase increased with increasing intensity of subthreshold stimuli and ranged from 91 to 148 percent of the Q-T interval. Subthreshold stimuli of a wide range of intensity could elicit a full response. Two types of supernormal phase, early and late in relation to the cardiac cycle, were observed. The latter was attributed to the summation of subthreshold stimuli with either spontaneous phase 4 depolarization of a ventricular ectopic pacemaker or atrial depolarization potentials. Its possible connection with Wedensky facillitation was suggested. The ventricle was less excitable after an ectopic beat than after a normally conducted beat.