The aim of the work: The study of the chronobiolgical rhythm of the blood acid-base balance in the period of adaptation of the new-borns in the first 72 hours of life. Materials and methods: The study was performed in 19 healthy new-borns from healthy women with a physiological pregnancy evolution during the first hour after birth and thereafter at the childrens’ age in 12 hours, 24 hours, 36 hours, 48 hours, 60 hours, 72 hours and the 5th day. The blood acid-base balance was appreciated with the help of the device “MicroAstrup” with the determination of the following parameters: actual pH, oxygen pressure (pO2mmHg), CO2 pressure (pCO2mmHg), actual bicarbonate (ABmmol/l), standard bicarbonate (SBmmol/l), tampon base (BBmmol/l), base excess (BEmmol/l), CO2 (mmol/l) and O2 (%) blood concentration. Results: The performed study in healthy new-borns in the first 60 minutes after birth have proved the presence of decompensated metabolic acidosis in blood, which is confirmed through decreased actual pH, high base deficit, low level of actual, standard and tampon bicarbonates, decreased CO2 pressure, diminished blood pressure and concentration of oxygen.Conclusions: In the period of postnatal adaptation of the healthy new-borns the acid-base blood balance is submissed to a chronobiological rhythm which is characterized through the decrease of compensated metabolic acidosis from the age of 12 hours toward 24 hours and 36 hours, and from 60 hours till 72 hours; as well the increase of compensated metabolic acidosis around the age of 48 hours with the subsequent normalization on the 5th day of life.