To more accurately describe the physical nature of the pulmonary vasculature, pulsatile pulmonary blood pressure and flow were studied in 12 unanesthetized dogs. Transmural pulmonary artery pressure was measured with micromanometer-tipped catheters. Blood flow was determined using implanted electromagnetic flow probes. Fourier analysis was performed on data, which had undergone a filtering procedure, to eliminate unwanted noise from the system. The resultant computer output was expressed as input impedance and phase. Physiological linearity of the pulmonary vasculature was validated by analyzing data at varying heart rates and blood flows. These perturbations yielded impedance and phase curves virtually superimposable in individual animals. With this method of data analysis vascular resistance can be expressed as the ratio of mean pressure and flow while an estimate of the physical state and dimension of pulmonary vessels can be derived by averaging high-frequency impedance moduli. Because previous studies had reported a wide variability in that portion of the impedance curve used to determine the high-frequency average or characteristic impedance, several portions of the impedance spectrum were analyzed for all dogs. On the basis of these data it appeared that moduli average over the range of 7-11 Hz best fit the criteria for characteristic impedance. Serotonin infusion markedly increased both pulmonary vascular resistance and all impedance moduli suggesting stiffening and decreased volume in both small and large vessels. Isoproterenol infusion markedly increased rate, flow and pressure but did so within the framework of a normal impedance spectrum, verifying the linear nature of the pulmonary circulation at high right ventricular outputs.