Pulmonary arterial pressure and flow, and pulmonary capillary flow have been measured in 13 dogs, breathing air. These parameters were also measured during ventilation with 10% oxygen in nitrogen, and during an intravenous infusion of serotonin. Pulmonary arterial distensibility and compliance were calculated from the recorded pressure-flow data. While the animals breathed air, distensibility and compliance averaged 3·32%/cm H2O and 1·42 ml./cm H2O respectively. These values were greatly reduced during hypoxia and serotonin infusion – both of which conditions led to an increase in pulmonary arterial resistance. Under physiological conditions, the time constant (CR) of the pulmonary arterial system remained constant over a wide range of pulmonary arterial resistance.