To establish the incidence and course of right ventricular (RV) conduction delay during the regression of right ventricular dominance, 12 serial ECGs were recorded at regular intervals until age 36 months in a cohort of 43 normal neonates. Additionally, ECGs were recorded similarly in 6 infants from the age of 1 month and in 7 from 2 months. The right precordial R waves were classified according to their amplitudes and their occurrence before or after the S wave. QR duration and its ratio to QRS were measured in right and left chest leads. The incidence of notched R waves, which is very rare in neonates, increased after the first month and occurred before the peak of R. Thereafter, the height of the notch decreased and then occurred after the peak of R. This turned into an r' (after the S wave) and finally the amplitude of r' decreased further and evolved into a notched S pattern. An rSR' or qRS pattern in the right precordial leads was never observed. An abnormal incomplete right bundle branch block during infancy can be distinguished from physiological right ventricular conduction delay by the unvarying amplitude of the R' wave, preceded by an S wave.