Pulmonary arterial input impedance spectra were computed in goats in whom the appropriate pressure and flow transducers had been chronically implanted. In response to either hypocapnia or hypercapnia, under anesthesia (1% halothane in a 70% nitrous oxide--30% oxygen mixture) there were no significant modifications of impedance at zero frequency; no consistent or significant changes in the impedance moduli at frequencies between 2 and 14 Hz were observed; the position of the first impedance minimum or the subsequent maximum was not modified; however, pulmonary vascular resistance increased significantly with hypercapnia. Although the load opposing right ventricular ejection was not modified by variation of Paco2, right ventricular work was reduced in response to hypocapnia and augmented in response to hypercapnia.