The aim of the work: The study of the 24 hour rhythm of the acid-base balance in the period of physiological pregnancy and after birth in healthy women. Materials and methods: The studies were performed on 22 healthy women in the period of 38-40week physiological gestation and in the first day after the physiological birth, at 11a.m, and 11 o’clock in the evening, and the 5th day after birth at 11a.m. The acid-base balance of blood was assessed with the help of the device “Micro-Astrup” determining the parameters: actual pH, CO2 pressure (mmHg), O2 pressure (mmHg), actual bicarbonate - A.B (mmol/l), standard bicarbonate-SB (mmol/l), tampon base-BB (mmol/l), base excess-BE(mmol/l), CO2 concentration (mmol/l), and O2 (%) in blood. Results: The acid-base balance investigations of blood in healthy pregnant women have demonstrated at 11 o’clock in the morning the presence of the compensated metabolic acidosis confirmed by actual pH within normal limits, moderate base insufficiency, low level of actual bicarbonate, standard bicarbonate and tampon base, decreased CO2 pressure in blood, and total normal CO2 blood concentration, low O2 blood pressure, and O2 concentration – within underneath norms. Conclusions: The acid-base balance of blood in the period of physiological pregnancy and after birth in healthy women is subjected to a strictly determined chronobiological rhythm which is expressed through compensated metabolic acidosis during the day at 11a.m. and compensated respiratory alkalosis at night at about 11p.m.