The structure and distribution of pulmonary lymphatics and their permeability to fluids and particulate materials have been investigated in the lungs of rats following fixation by combined intratracheal and vascular perfusion. In such preparations, the lymphatics remain in a distended state, and a close relationship to other structural components of the pulmonary interstitium is maintained. They were identified in regions with an abundant amount of connective tissue, forming an eleborate plexus within the pleura, the interlobular septum, peribronchial and perivascular areas. Recent data have shown that water-soluble molecules and particulate matter are removed from the interstitium along the lymphatic capillary (initial lymphatics) segment. It is distinguished by attenuated endothelial cells with extensively overlapping cell margins which are easily separated. We have studied this segment of the lymphatic vascular system following intratracheal injections of colloidal particles (ferritin and carbon) to determine the structural features responsible for the transport of large molecules and particulate materials across the lymphatic endothelial wall in the lung. The results showed that the tracer particles cross the lymphatic endothelial wall via the clefts of intercellular junctions. While the tracer particles were observed within vesicles, the question of transport across the lymphatic endothelium via plasmalemmal vesicles is still not settled since the number and size of vesicles containing tracer particles also increased with time. Intravascular injected dextran was also localized within the clefts of intercellular junctions and plasmalemmal vesicles. The results obtained with intratracheal and intravascular injected tracer substances are consistent with those observed in lymphatic capillaries for other tissues.