Cardiac activity in rats during the postnatal period was studied in vitro and in vivo after exposure of rat pups to antenatal acute hypobaric hypoxia at the stage of organogenesis (day 9-10 of gestation). Cultured cardiomyocytes from rat pups exposed to antenatal hypoxia were characterized by increased rate of contractions and decreased reactivity to norepinephrine. Heart rate elevation, predominance of sympathetic influences on cardiac activity, and significant increase in norepinephrine concentration in the cerebral cortex were found in freely moving animals exposed to antenatal hypoxia. Our results indicate that hypoxia at the stage of organogenesis modulated cardiac activity during the postnatal period, which manifested at the level of effector structures in the heart and activity of regulatory systems.