P.a. chest X-rays were obtained during inspiration and expiration in 180 normal children aged 3-14 years. The mean and standard deviation for inspiratory and expiratory values of transverse, mid-right, mid-left and longitudinal diameters of the heart, cardiothoracic ratio (CTR), angle of inclination of longitudinal axis and cardiac area were analyzed for age and sex groups. A highly significant expiratory increase was found in the transverse and longitudinal diameters of the heart and in CTR. The expiratory increase in transverse diameter correlated much better with the angle of inclination of the longitudinal axis than with the level of the diaphragm. On the other hand, changes in CTR and in the width of the shadow of great vessels related more closely to the level of the diaphragm. The correlation between CTR and diaphragmatic level was significant enough to establish regression formulas and nomograms whereby a CTR at a certain diaphragmatic level can be estimated when the CTR at another level of diaphragm is known, providing a reliable method for comparing roentgenograms taken in different phases of respiration. The configuration of the heart was so affected in expiration that straitening of the left border, a wide vascular shadow, a prominent ascending aorta is the rule. Furthermore, with the elevation of the diaphragm, there was a significant loss of sharp outlines of pulmonary vascular images.