Abstract 1. 1. Serial electrocardiograms were taken on 88 premature infants from birth to 7 weeks of age. The tracings of the same age-group were subdivided into 4 weight-groups. 2. 2. Ten infants died; 8 of these contributed tracings only for the first age-group. Those electrocardiograms proved to be no different from the tracings obtained in the surviving infants, and were included in the total group for statistical purposes. 3. 3. Mean heart rate increased with age, from 125 at birth to 170 bpm at the oldest age-group studied, whereas the Q-T c was found to be relatively prolonged. Tall and pointed P waves were found occasionally in Leads II and V 1 , whereas the QRS voltage was frequently lower than that of normal full-term babies. 4. 4. The average P axis remained between 50 and 60 degrees for all age-groups. The maximal QRS and T axes presented a wide scatter in both the frontal and horizontal planes. The QRS axis was usually directed rightward, downward, and anteriorly, except for 4 babies in the first age-group in whom a marked posterior orientation was observed. The T axis was directed mainly leftward and posteriorly, forming an angle with the QRS axis which increased until the infants were 90 hours old, and which then gradually decreased with increasing age. The initial vectors of the QRS were oriented in an anterior or left lateral direction, becoming more anterior with age, whereas the final vectors tended to be situated in an opposite direction. 5. 5. The only findings that could be related to the weight of the infants were the significantly greater occurrence of a q in Lead V 6 , and of negative T waves in right precordial leads in babies who were lighter in weight. These changes probably reflect a greater degree of immaturity.