The fate of ink and viable lymphoid cells placed in the wall and the lumen of the cycling rat uterus was determined. India ink injected into the myometrium rapidly filled major lymphatic trunks, going to the ipsilateral renal and/or iliac lymph nodes, depending on the site of inoculation along the uterus’ length. No other abdominal or thoracic nodes seem to be involved. However, ink injected into the uterine lumen penetrated the endometrial stroma very slowly and only scattered dye-containing phagocytic cells could be seen in draining nodes. Ink-laden macrophages were retained for at least several days in the endometrium. Lymphoid cells similarly injected were traced by several techniques: 1) scintillation counting of tissues for ⁵¹Cr-labeled cells; 2) autoradiography for [³H]uridine-labeled cells; 3) fluorescent microscopy for fluorescein-containing cells in frozen tissues; and 4) graft-versus-host reactivity in draining lymph nodes. All techniques gave the same results: cells injected into the myometrium appeared rapidly in draining nodes, inducing a graft-versus-host reaction, whereas cells placed in the uterine lumen were found in only small numbers in the regional nodes, even after several days, and graft-versus-host reactions were minimal or absent. The barrier(s) to cell movement from the uterine lumen remains to be defined, but is of considerable interest since cell traffic in the other direction is a well-documented normal process. The combined effect of restricted cell entrance to the stroma, long retention of macrophages at this site, and an apparent paucity of lymphatic vessels in the endometrium most likely is to slow antigen handling from this site, which may help regulate maternal immune responses during pregnancy.