Controversy continues about the contribution of the veins to pulmonary vascular resistance. From data obtained in studies using intravascular catheters, it appears that a major fraction (up to 44%) of the total pulmonary vascular pressure drop resides in larger (greater than 1.0 mm diam) veins, whereas micropuncture data and various models give much less pressure drop. Theoretically, artifactual pressure drops can be obtained if an intravascular catheter partly obstructs the vessel. We made measurements of pressure in the same lung vein with two different-sized catheters (1.2 and 0.6 mm OD, respectively). In paired experiments the larger catheter always measured a higher pressure than the smaller one, except close to the large lobar vein outlet. In some of the experiments we measured the diameter of the vessel containing the indwelling catheter by freezing the lung and then serial-sectioned the frozen lung. From these data we could infer that the range of vein diameter in the which the smaller catheter measured a lower pressure was 1.5-4 mm. We conclude that the larger catheter overestimated the pressure because of greater obstruction. The pressures obtained with the smaller catheter suggest that little (less than 10%) of the total pulmonary vascular resistance resides in veins larger than approximately 1 mm diam under zone 3 baseline conditions.