Scanning electron microscopic observations on the luminal aspect of the high-endothelial postcapillary venules (PCV) of lymph nodes of mice, rats, hamsters, and guinea pigs show that the surface topography of these venules differs considerably from that of other venules. Two types of surface relief occur: one in which the endothelial cells bulge deeply into the lumen, thus presenting the appearance of a “cobblestone” pattern and another in which the endothelial cell has plate-like lateral extensions which intertwine with similar extensions of other cells, thus presenting a surface relief of “interlacing plates.” Both patterns occur in the same PCV. The part of the PCV with the interlacing plate pattern is located distally from the section with the cobblestone pattern and continues directly into the smooth-walled larger venules. Lymphocytes enter the wall of the PCV through the cell body of the endothelial cells regardless whether the luminal relief has a cobblestone or an interlacing plate configuration. To a more limited extent, transmural passage of lymphocytes occurs through the walls of larger and smooth-walled venules with low endothelium. The direction of movement of lymphocytes at such sites is not known.