Castiglioni, P., R. Tommasini, M. Morpurgo, and M. Di Rienzo. Modulation of pulmonary arterial input impedance during transition from inspiration to expiration. J. Appl. Physiol. 83(6): 2123–2130, 1997.—We investigated whether respiration influences pulmonary arterial input impedance during transition from inspiration to expiration in five anesthetized, spontaneously breathing dogs. Impedance (Z) was separately assessed for heart beats occurring in inspiration, in expiration, and during the transition from inspiration to expiration (transitional beat). Transitional beats were scored by the ratio between the fraction of beat falling in expiration and the total beat duration [expiratory fraction (E_fr)] to quantify their position within the transition. In transitional beats, input resistance linearly increased with E_fr; Z modulus at the heart-rate frequency (f_HR) decreased up to −50% for E_fr = 50%. Z phase at f_HR was greater than in inspiration for E_fr<40% and lower for E_fr >50%. Unlike blood flow velocity, mean value and first harmonic of pulmonary arterial pressure were correlated to E_fr and paralleled the changes of input resistance and Z at f_HR. This indicates that respiration influences Z through modifications in arterial pressure. The evidence of important respiratory influences on Z function may help the pathophysiological interpretation of dysfunctions of the right heart pumping action, such as the so-called cor pulmonale.