Immunohistochemical detection of matrix metalloproteinases (MMP) 2 and 9 was carried out in the myocardium of the left ventricle of prematurely born (21 and 21.5 days of gestation) and full-term (22 days of gestation) rats at 42, 56 and 180 days of the postnatal period of ontogenesis. The intensity of immunopositive staining was assessed by a scoring system. Preterm birth leads to an increase in the intensity of the immunopositive reaction to MMP-2 and MMP-9 in the wall of the left ventricle of rats. An increase in the intensity of the MMP-2 positive reaction in the left ventricle of rats is observed the earlier, the greater the degree of prematurity. The intensity of the MMP-9 positive reaction in the left ventricle of male rats is greater with a more pronounced degree of prematurity. An increase in the intensity of the MMP-2 and MMP-9 positive reaction in the wall of the left ventricle of female rats due to preterm birth is determined exclusively in animals born on the 21st day of pregnancy, that is, with a greater degree of prematurity.