Morphological and cytochemical features of the heart were studied in rats aged 1, 14, 21, 30, and 45 days born of animals which were given merkazolil (3 mg per 1 kg of body weight) during pregnancy and lactation. The relative heart weight of the progeny of the experimental animals proved to be considerably less during the postnatal period under study in comparison with controls. A number of structural and cytochemical changes were displayed by the myocardium pointing to its dysfunction.