Pregnant rats were treated with 5-bromo-2'-deoxyuridine (BrdU) from embryonic d 12 (ED12) to ED14. BrdU administration allowed timed labeling of dividing embryonic cells in utero, since the drug is incorporated into the DNA in place of thymidine during the S-phase of the cell cycle. ED14 rat cerebral cortex or placenta was grafted into the brain of adult rats. Anti-bromodeoxyuridine immunohistochemistry was used for identifying labeled transplanted cells after different survival periods in paraffin-embedded sections. BrdU labeled cells were observed in both intraventricular and intraparenchymal cortical grafts, even after a 3-mo survival. Although the percentage of positive cells decreased in comparison with ED14 cortex, the level of BrdU (i.e., the intensity of anti-BrdU immunohistochemistry) in labeled nuclei was probably the same. BrdU pretreatment of ED14 cells prior to grafting did not affect the proliferative ability of the grafted tissue. In ED14 placental grafts, all trophoblastic cells were labeled distinctly. This precise labeling technique enabled an examination of individual migrating trophoblastic cells. However, migration of these cells into the host brain was very limited.