The postnatal growth of the lung was quantitatively investigated in rats aged 1 , 4 , 7, 10, 13, 21,44 and 131 days by light and electron microscopic morphometry .Lung volume (V,) increased first directly with body weight (W). After day 10 V, followed the function WO.?O. Based on the quantitative findings the postnatal lung growth could be divided into three phases.