Body weight, absolute and relative heart weight, DNA-synthetic activity of cardiomyocytes, and number of nucleoli in cardiomyocyte nuclei were reduced in newborn albino rats exposed to antennal hypoxia. All these changes developed against the background of oxidative stress. In mature animals, the absolute and relative heart weights were decreased, but the ratio of mononuclear cardiomyocytes in both ventricles was higher than in the control. These changes were accompanied by an increase in the percent of collagen fibers in the myocardial stroma. In the right ventricle, an increase in the cardiomyocyte length to width ratio and a decrease in the number of nucleoli were found. The observed changes can probably induce heart function disturbances under pathological conditions.