Heath, D., Smith, P., Williams, D., Harris, P., Arias-Stella, J., and Kruger, H. (1974).Thorax, 29, 463-471. The heart and pulmonary vasculature of the llama (Lama glama). A qualitative and quantitative histological study was made of the pulmonary trunk and muscular pulmonary arteries of a male and a pregnant female llama born and living at an altitude of 4,720 m above sea-level in the Peruvian Andes. A similar study was made on the fetal llama. The individual cardiac chambers of the two adults were weighed. Our results show that in the adult llama there is no hypertrophy of the right ventricle or of the media of the pulmonary trunk or small pulmonary arteries. This appears to be of evolutionary significance in respect of survival at high altitude and suggests that the llama does not have a sustained significant pulmonary hypertension. The pulmonary arteries of the fetal llama are thick-walled and we associate this with the physiological pulmonary hypertension of fetal life.