Abstract 1. 1. Simultaneous phonocardiograms and electrocardiograms were taken serially in 61 newborn infants, in order to study the relationship of the auscultatory findings to the electrocardiographic alterations known to occur during the first few days of life 2. 2. A second group of 19 infants was studied more intensively by phonocardiography, using tape recording methods, during the first 8 hours of life. 3. 3. Electrocardiograms during the first 72 hours of life showed: (a) tendency for peaking of the P waves to appear; (b) anteriorly directed ÂT which rapidly rotated to a posterior position; and (c) rightward ÂQRS which remained constant. 4. 4. Phonocardiograms in data derived from both groups showed: (a) an early systolic click appearing almost universally at birth but with diminishing frequency later—its timing averaged 39.7 msec. after the onset of the first sound; (b) a second systolic click in some subjects, averaging 63.9 msec. after the onset of the first sound; (c) narrowly split second sound at birth with a tendency to a wider split later (average 19.4 msec. at 6 to 8 hours of life); (d) transient low-intensity systolic murmurs noted in approximately one third of the infants. These were of three varieties: the most common type was a decrescendo murmur in the pulmonic area; less commonly seen were crescendo-decrescendo murmurs; and rarely (4 instances) the timing was in late systole. No diastolic murmur was seen. 5. 5. The probable relationship of the electrocardiographic and phonocardiographic findings to the alterations in circulation that occur at birth are discussed.