The authors followed in dynamics the systolic arterial blood pressure and morphometric changes in the myocardium (cardiac, index, mean number of myocardial cells per unit of area c I mm2, mean diameter of the myocardial cells of the papillary muscles and subendocardial laer of the left ventricle) in thymectomized and sham thymectomized rats. The studies were made after thymectomy and sham thymectomy on the second, sixth, twelfth and sixteenth week. The systolic arterial blood pressure in thymectomized rats was elevated gradually and reached up to 170 mmHg on the sixteenth week, but it was in normal ranges in sham thymectomized animals. The cardiac index and mean diameter of the myocardial cells were larger in thymectomized rats, but the mean number of cells was smaller in comparison with sham thymectomized animals-results showing development of myocardial hyperthrophy after thymectomy. The induced myocardial changes were probably of adaptive character and could be due to neurohormonal mechanisms and hemodynamic loading of the heart becuase of the elevated arterial blood pressure after thymectomy.