The product of resistance, R, and compliance, C, (RC-time) of the entire pulmonary circulation is constant. It is unknown if this constancy holds for individual lungs. We determined R and C in individual lungs in CTEPH patients where resistances differ between both lungs. Also, the contribution of the proximal pulmonary arteries (PA) to total lung compliance was assessed. Patients (n=23) were referred for the evaluation of CTEPH. Pressure was measured by right heart catheterization, and flows in the main, left and right PA, by MRI. Total, left and right lung resistances were calculated as mean pressure divided by mean flow. Total, left and right lung compliances were assessed by the pulse pressure method. Proximal compliances were derived from cross-sectional area change A and systolic-diastolic pressure difference P (A/P) in main, left and right PA, multiplied by vessel length. The lung with the lowest blood flow was defined "Low-Flow" (LF), the contralateral lung "High-Flow" (HF) . Total resistance was 0.57±0.28mmHg•s/ml, and resistances of LF and HF lungs were 1.57±0.2 vs. 1.00±0.1mmHg•s/ml, respectively, p<0.0001. Total compliance was 1.22±1.1ml/mmHg, and compliances of LF and HF lung were 0.47±0.11 and 0.62±0.12ml/mmHg respectively, p=0.01). Total RC-time was 0.49±0.2s, and RC-times for the LF and HF lung were 0.45±0.2 and 0.45±0.1s respectively, not different. Proximal arterial compliance, given by the sum of main, right and left PA compliances, was only 19% of total lung compliance. The RC-time of a single lung equals that of both lungs together, and pulmonary arterial compliance comes largely from the distal vasculature.