The pathway of migration of lymphocytes across the walls of the postcapillary venules was studied in lymph nodes of rats. By means of serial sections, it was determined that lymphocytes enter the venule wall through the endothelial cells and not through the intercellular spaces. After the endothelial cell has been penetrated, the lymphocytes enter the intercellular spaces, from where they gain access to the lymphatic parenchyma. It cannot be excluded, however, that in some instances the path of migration of a lymphocyte is entirely through an endothelial cell from lumen to subendothelial space. While perforating an endothelial cell, a migrating lymphocyte may be in simultaneous contact with the vascular lumen, the intercellular space, and the subendothelial tissue.