C. J. Beinlich, C. J. Rissinger and H. E. Morgan. Mechanisms of Rapid Growth in the Neonatal Pig Heart, Journal of Molecular and CellMar Cardiology (1995) 27, 273–281. During the first 2 weeks of life the left ventricular free wall of the neonatal pig heart grows rapidly. The mass of the left ventricular flee wall (LVFW) increased from 2.22±0.10 g to 9.62±1.01 g while the right ventricular free wall (RVFW) increased from 2.03±0.24 g to 3.56±0.41 g from birth to 14 days of age. During the same period, the cellular volume of myocytes from the LVFW increased from 1075 μm³ to 3688μm³ while myocytes from the RVFW increased in volume from 1511 μm³ to 2454 μm³. The number of RVFW myocytes did not change during the first 2 weeks of life, while the number of LVFW myocytes increased 28%. Myocytes from both ventricles were approximately 90% mononuclear from birth to 4–5 days of age. By 14 days, 67% of LVFW myocytes and 53% of RVFW myocytes were multinucleated. When growth of the heart was restrained by treatment of the piglet with enalapril maleate, the LVFW mass was reduced by 24% over 2 weeks compared to hearts from untreated piglets and was accounted for by a reduction in myocyte volume. Enalapril treatment did not alter the number of myocytes in either the LVFW or RVFW as compared to hearts from untreated piglets. After 14 days of enalapril treatment, the percentage of multinucleated cells was reduced in the LVFW and unchanged in the RVFW as compared to hearts from untreated piglets. These results indicate that cellular hypertrophy accounts in large part for the rapid growth of the LVFW of the neonatal pig heart and that enalapril reduced normal growth of the left ventricle by reducing left ventricular myocyte hypertrophy.