Atrial natriuretic peptide (ANP) was assayed on the 1st, 3rd, 7th and 10th day of extrauterine life and the levels of the hormone were checked after the 2nd month in 12 infants. After an initially rapid increase, with a peak of the 3rd day (45.7 +/- 7 pg/ml the 1st day of extrauterine life and 175 +/- 5 pg/ml on the 3rd day), we can recognise a progressive decrease until the measurement made at 2 months (61.5 +/- 5 pg/ml). The reported data show the important role of ANP in adaption of life after birth. This can be considered as a compensatory response because it represents an attempt to maintain the balance of positive sodium which is essential for the growth of the newborn.