The littoral cells of the sinuses in the popliteal lymph nodes have been studied in Wistar rats under physiological conditions of hemodynamics and 1 h after ligation of the caudal vena cava. Intercellular clefts (1-4 mcm wide) are demonstrated to exist between the littoral cells of the internal wall of the marginal sinus. Presence of fenestrae in the peripheral thinned areas of cytoplasm is specific for these cells. The diameter of the fenestrae is 60-1,000 nm however, the fenestrae with the diameter up to 100 nm predominate. Lining of the medullar sinuses is also characterized by a high content of fenestrae, the maximal diameter of the fenestrae being the same as in the marginal sinus, although most of the fenestrae have the diameter 50-500 nm. No open intercellular clefts between the littoral cells of the medullar sinuses are revealed. In some cases, penetration of lymphocytes through the lining of the sinuses is observed. Under venous congestion, the diameter and amount of the fenestrae, size and amount of open intercellular clefts in the lining of the lymphatic sinuses increase significantly. This demonstrates its increasing permiability, having an essential importance for compensation of circulatory disturbances in the zone of the venous congestion.