In a series of 24 subjects who ranged in age from 2.5 months to 50 years and who were not suffering from cardiac or pulmonary disease, the density of muscularized pulmonary arteries was established in relation to their diameter. The number of arteries per unit area of lung tissue was corrected to account for various states of lung expansion of or collapse. The density of pulmonary arteries decreased rapidly during the first few years of life, then more gradually during the first two decades of life. Concomitantly, the diameter of the smallest muscularized arterial branches increased with age. These findings indicate that, even after completed development of lung tissue, there is a retraction of media in the sense that small arterioles lose their media, a process that continues until early adulthood. The values thus obtained may serve as a standard for comparison when the pulmonary arterial density is studied in cases of pulmonary hypertension in which it reportedly is greatly diminished.