The normal structure and pattern of branching of the pulmonary arteries of the rat have been described and quantified, using precise morphometric techniques, after injection of the pulmonary arteries with a radio-opaque medium. Rats of the same strain from three different sources have been compared, and similarities between rat and man have been assessed. In rat, as in man, the pulmonary artery accompanies the airway and branches with it and, in addition, there are extra arterial branches. Most arteries within the rat lung are muscular and have no more than two central elastic laminae. The percentage wall thickness of the muscular arteries is between 1 and 3%, similar to that found in adult human lungs. At the periphery of the arterial tree there is a mixed population of muscular, partially muscular and non-muscular arteries. The distribution by size is similar in man and rat, but there are few wholly muscular arteries within the acinar region in the rat. The similarity of the features of the pulmonary arteries of rat and man makes the rat a suitable experimental animal for study of changes in pulmonary hypertension. However, any change must be interpreted in relation to arterial structure in normal rat lungs from the same source, since some significant differences have been found between rats from different sources in respect of wall thickness and number of vessels.