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We have registered, on 36 normal newborns at term, ECG recordings of sixty seconds at a speed of 100 mm/second, from birth tooth sixth month of postnatal life. The heart rate variability was measured on its double slope, short and long term by the numerical method that Yeh proposed, consistent with the "differential index" (DI) and the "interval index" (II). For the short term variability, we have obtained minimum values at birth and maximum values at 72 hours of postnatal life, with a tendency to drop up to the end of the first month and later to become stable until the end of six months of age. The changes found among different ages, could be related to modifications in basal heart rate under autonomic nervous system control. The variability values of the long term, do not show a clear age relation.