His bundle potentials were recorded in 50 children. Eight had normal hearts, 11 had ventricular septal defects, seven had the tetralogy of Fallot, seven had secundum atrial septal defects, and eight had pulmonary valve stenosis. There were miscellaneous defects in nine children. Ages ranged from 2 weeks to 14 years. Two intervals were measured on recordings: (1) from the initial atrial activity (P′) to the His bundle potential (H), the P′-H interval, and (2) from H to the initial ventricular activity (V), the H-V interval. The H-V interval lengthened with age. The P′-H and H-V intervals did not differ significantly when different cardiovascular lesions were compared.