Pulmonary artery distensibility was studied with spin-echo magnetic resonance imaging in 20 normal subjects of variable age and in four patients with pulmonary arterial hypertension. The distensibility was found to be significantly lower (8%) in patients with pulmonary arterial hypertension than it was in normal subjects (23%). No age-related difference occurred. Magnetic resonance velocity mapping of the pulmonary artery blood flow was performed in 26 normal subjects--11 had mapping in the mid pulmonary artery, 15 had mapping in the distal pulmonary artery, and mapping in the four patients with pulmonary arterial hypertension was in the mid pulmonary artery. The pulmonary artery flow volume was compared with aortic flow and left ventricular stroke volume and a very good correlation was found. A retrograde flow of 2% occurred in the normal subjects serving to close the pulmonic valve. Antegrade plug flow occurred in most normal subjects but varied among individuals. There were also other variations in the flow pattern among normal individuals. All patients with pulmonary arterial hypertension had a markedly irregular ante- and retrograde flow and a large retrograde flow (average 26%). Magnetic resonance imaging offers a noninvasive way to evaluate pulmonary arterial hypertension as well as to quantitate pulmonary and aortic flows in, for example, left-to-right shunts.