Postcapillary venules of mouse lymph nodes are vessels situated between capillaries in the cortex and paracortex and efferent veins in the hilar region. They are characterized by a high endothelium. Their vessel diameters and periendothelial ultrastructure are similar to these same elements in the postcapillary venules of other tissues. The high endothelial cells are variable in shape. Their well-developed organelles are indicative of intense synthetic activity. Distinctive particles, multivesicular bodies with dense matrices, are associated with a prominent Golgi complex. An endothelial cell coat is present at the luminal surface. Many lymphocytes, singly or in groups, are within the venule walls. The majority have traversed the endothelium and lie adjacent to the pericytes, which constitute the major obstacle to lymphocyte emigration from the venules. Most lymphocytes that have not yet passed the endothelial layer lie between two or more endothelial cells. Analysis of serial sections supports the contention that lymphocytes pass between, rather than within endothelial cells.