The histologic pulmonary vascular changes and the pulmonary arterial pressure changes which occur during the first 13 weeks of life have been studied in mongrel puppies. The histologic changes were based on measurements of the average lumen to wall ratios of arterial vessels 50 to 200 μ in diameter. The pulmonary arterial pressure changes were recorded by direct needle puncture of the main pulmonary artery in anesthetized thoracotomized animals. The average L/W ratios of all neonatal mongrel puppies from separate litters were less than 4.0, the boundary chosen to distinguish neonatal and adult pulmonary arterial vasculature. All puppies younger than 1 month had average L/W ratios below 4.0. Although average mean pulmonary arterial pressures were found to be near adult levels by age 1 week, the ratio of pulmonary to systemic arterial pressure, expressed as a percentage, did not consistently approach adult levels until 1 month of age. The dimensional evolution of small pulmonary arteries and the gradual decline in the ratio of pulmonary to systemic pressure toward near adult levels at 4 weeks suggested a cause-and-effect relationship between these 2 phenomena. Major operative procedures have been performed on puppies 3 to 5 weeks of age with survival. It is proposed that experimental pulmonary hypertension produced in 4-week-old mongrel puppies would closely approximate the situation of a high pressure congenital cardiac shunt in humans without excessive operative mortality.