Triple resin casts were made of the pulmonary arteries, veins, and airways from six dog lungs. The airways were cast at a pressure of 25 cm resin in all six. In the first three, both vessels were cast at a pressure of 30 cm resin, and in the second three, arteries were at 10 cm resin and veins at 5 cm resin. Measurements were made of luminal airway diameters down to 1 mm and of the luminal diameters of the corresponding segments of the vascular trees. The relation of one to the other was shown by calculating the regression lines for the corresponding diameters. Intrapulmonary arteries and veins are of approximately equal diameter when cast at 30 cm resin, while the veins are 20% larger than the arteries when cast at 5 cm and 10 cm of resin, respectively. Both vessel diameters are 75% of bronchial at the higher pressure, while at the lower pressures arteries are 59%, and veins 71% of bronchial. In any individual, vessel diameters are a relatively constant proportion of airway diameter. This constant of proportionality varies considerably between dogs, its value ranging from 0.48 to 1.03. It is concluded that in any individual all three trees have similar diameter ratios--that is, the ratio between mean diameters of branches in successive orders.