The postnatal development of high-endothelial venules (HE-venules) in popliteal lymph nodes of normal and athymic mice was studied by light and electron microscopy. In the normal mice, undifferentiated postcapillary HEvenules are already present at birth. During the first four days after birth, ribosomes and mitochondria in the endothelial cells of the venules become more numerous, and the endoplasmic reticulum and in particular the Golgi complexes enlarge. At day four lymphocyte diapedesis through the venules becomes apparent. From the tenth day the HEvenules are structurally mature. However, diapedesis activity still increases till about a month postnatally. In mice with congenital aplasia of the thymus (nude mice) and in neonatally thymectomized mice the development of the HEvenules is similar to that in normal mice. In the athymic mice both lymphocytes and granulocytes migrate through the walls of the HEvenules. However, the blood level of lymphocytes in these mice is far below the value in normal mice. It is concluded that the development of the HEvenules during the first month of life is independent of the blood level of lymphocytes.