The mean wall thickness, in relation to the external diameter, of 100 muscular arteries distended by barium gelatin at 100cm H2O pressure was calculated in "normal" lungs from patients who ranged in age from 3 to 79 years. For normal vessels less than 2 mm in diameter the mean was 2.4% of the external diameter. The mean for vessels in the five persons under 40 years was 1.8% but it was 2.8% for the five older individuals. Although this difference may reflect some increased muscularity with age, some of the increment may be related to a reduced distensibility of vessels caused by the accumulation of interstitial fluid, as manifested by increased weight in the older lungs. Intimal fibrosis was found to increase with age but occurred in less than 50% of the vessels measured.