The present study examines by freeze fracture the structure of endothelial Intercellular junctions in pulmonary intra-acinar arteries and veins. Because of the complexity of the pulmonary vascular bed, it was necessary to devise a means to distinguish venous from arterial vessels in freeze fracture replicas. By modifying a recently described technique, which entails occluding venous outflow during intravascular pcrfusion fixation of the lung, the arteries were emptied of and the veins filled with erythrocytes. Tight junctions (TJ) of intra-acinar arteries, ranging in internal diameter from 17 to 125 fun, consisted of two to six continuous, interconnected rows of particles, 100-500A in length, present in grooves on the E face and complementary low, particle-poor ridges on the P face. Numerous large gap junctions (GJ) were present in the meshwork of TJ and formed rows parallel to the TJ elements. In intra-acinar veins, measuring 32-284 pan in internal diameter, TJ consisted of one to five sparsely interconnected rows of particles on the E face and low, virtually particle free ridges on the P face. Gap junctions were smaller and fewer in veins than in arteries. In both arteries and veins," special endothelial junctions" were observed. These consisted of closely spaced, alternating rows of GJ and TJ particles and were similar to those observed in systemic vessels (Simioneecu et aL, 1976.) The presence of GJ suggests that there is extensive electrotonic and/or metabolic coupling in arterial endothelium, and that this is less extensive in venous endothelium. The continuous network of multiple TJ particles in both arteries and veins suggests that these segments are less permeable than the venular ones.