1. The influence of mean pulmonary arterial pressure (mean P(pa)) on dynamic (C(d)) and pseudostatic compliance (C(ps)) of the pulmonary artery was studied at a constant and a changing heart rate. C(d) is the change in cross-sectional area (CSA) relative to the change in P(pa) throughout a heart cycle. C(pa) is the change in mean CSA relative to the change in mean P(pa). If C(d) is known, pulmonary blood flow can be computed from the P(pa) using a windkessel model. We investigated whether C(ps) can be interchanged with C(d). 2. In nine anaesthetized pigs, we determined the mean CSA and C(d) of the pulmonary artery at various P(pa) levels, ranging from approximately 30 to 10 mmHg, established by bleeding. Two series of measurements were carried out, one series at a spontaneously changing heart rate (n = 9) and one series at a constant heart rate (n = 6). To determine CSA a conductance method was used. 3. C(ps) depended on pressure. The mean CSA versus mean P(pa) curves were sigmoid and steepest in the series with the increasing heart rate (established by bleeding). The CSA versus P(pa) loop during a heart cycle, giving C(d), was approximately linear and almost closed. The C(d) versus mean P(pa) relationship was bell shaped. Its width was smaller if the heart rate increased during the series of measurements. The pressure, where C(d) was maximum, was higher at higher heart rates. Furthermore, the maximum C(d) was not affected by the heart rate. 4. Because the pulmonary artery constricts with increasing heart rate, C(ps) will be overestimated during procedures where heart rate increases. C(d) should be determined on a beat-to-beat basis to calculate flow because it changes with mean pulmonary arterial pressure and heart rate.