Postcapillary venules in rabbit tonsils were studied by scanning (SEM) and transmission electron microscopy (TEM). Tonsils were perfused with Ringer solution and a glutaraldehyde fixative and blood cells were drained away leaving the migrating lymphocytes in the vascular wall. Specimens were freeze-cracked and the luminal surface of the venules were exposed for examination by SEM. The postcapillary venules under the SEM were lined by endothelial cells conspicuously and irregularly bulging into the lumen. The endothelial cell possessed columnar marginal processes, which were interdigitated at the base of the endothelium with those of adjacent cells, while they often formed intercellular bridges on the luminal surface. Migrating lymphocytes observed on the endothelium by SEM were located on the endothelial cell boundary region or plugged between the adjacent endothelial cells. Lymphocytes incorporated into the endothelial wall were found in the intercellular space by TEM. Careful analysis of SEM and TEM images led to the conclusion that they pass through the intercellular space in the endothelial wall. Migrating lymphocytes on the endothelium were all small and classified into 3 types on the basis of their surface architecture (occurrence of each type given in parenthesis): villous lymphocytes (12.5%), relatively smooth lymphocytes (59.1%), and smooth lymphocytes (28.4%). Relatively smooth and smooth lymphocytes formed the great majority of the migrating lymphocytes and they were tentatively identified as t [thymus-derived] lymphocytes.