Conditionally immortalized (temperature-sensitive) striatal-derived neuronal progenitor cell lines and primary neuroepithelial cells were transplanted into the CNS of gestational day 15-16 rat fetuses using an 'in utero' surgical procedure. Each fetus received 2.5-3 x 10(4) donor cells previously labelled in vitro by incubation with 5-bromo-2'-deoxyuridine (BrdU). At 5 days following transplantation, 69% of the fetuses were still alive. Engrafted cells were detected by BrdU immunohistochemistry, and the appearance of the engrafted cells and the time course of Nestin and PCNA expression were measured at 6, 24, 64 h and 5 days after transplantation. The evolution of Large T-Antigen immunoreactivity in engrafted temperature-sensitive (ts) cells was also evaluated at the above time intervals. The results indicate that the majority of the implanted cells were aggregated into clusters 24 h after transplantation. These clusters were not visible at 6 h, when most of the cells were isolated. The clusters were located in both the ventricles and parenchyma. These findings were common to both ts cells and striatal primary neuroepithelial cells. At 64 h and 5 days, isolated cells associated with the germinal layer and scattered throughout the parenchyma were also found. In the clusters, Nestin expression decreased proportionally with time following transplantation. Furthermore, Large T-Antigen immunoreactivity disappeared from ts cells between 6 and 24 h after transplantation. Finally, measurements of the temporal evolution of PCNA expression within the clusters indicate a progressive reduction in the mitotic activity of the transplanted cells. The results demonstrate that striatal primary neuroepithelial cells and conditionally immortalized neuronal progenitors can survive, migrate and/or compartimentalize into clusters whilst changing their antigenic properties and ability to proliferate.