We have reported the longitudinal distribution of vascular resistance in the three segments (arteries, microvessels, and veins) of the pulmonary circulation of adult rabbits (J Appl Physiol 60:539-545, 1986). To determine the longitudinal distribution of vascular resistance in lungs of younger animals, we measured microvascular pressures in the subpleural microcirculation in nine isolated blood perfused lungs of 3- to 4-wk-old rabbits. We used micropipettes and the servonull pressure measuring system to directly measure pressures in 20-50 µm diameter subpleural arterioles and venules. We also measured pulmonary arterial and left atrial pressures and lung blood flow. To determine the influence of vascular tone on segmental vascular resistance, in four lungs we measured microvascular pressures both before and after paralyzing the vasculature with papaverine. We found that 51% of the total pressure drop in the pulmonary circulation was in arteries, 24.5% in microvessels, and 24.5% in veins. Vascular tone contributed minimally to baseline arterial resistance. The greater arterial and venous resistance in lungs of 3- to 4- wk-old rabbits as compared to that in adult rabbits is probably due to differences in vessel geometry.