The ratio of right to left ventricular weight (0.3), and the ratio of the medial thickness of the pulmonary trunk to that of the aorta (0.4 to 0.7) are similar in humans and other mammals for which data are available. There are interspecies differences in quantity and arrangement of collagen, smooth muscle, and elastic tissue in the pulmonary trunk. The medial thickness of muscular pulmonary arteries is similar in man, baboon, ferret, goat, llama, and monkey. In most other mammals these vessels are more muscular than those in humans. The goat is similar to man because the muscular pulmonary arteries terminate at an external diameter of 100 microns. In most other mammals, smooth muscle is present in much smaller pulmonary arterial vessels. There is pronounced interspecies variation in the course and structure of the pulmonary veins. The cat, civet, dog, ferret, fox, goat, horse, monkey, and rabbit have pulmonary veins with thin fibrous walls such as occur in humans. However, the pulmonary veins are muscular in the cow, guinea pig, llama, pig, and rat.