It is well known that 5-bromodeoxyuridine (BUdR) when injected into pregnant animals may cause exencephaly, cleft palate, and limb abnormalities. Similarly, it is well established that the drug when added to a culture medium may prevent differentiation of embryonic cell systems without affecting cell division or cell viability. The goal of our experiments was to examine whether the congenital malformations resulting from BUdR treatment were due to lack of differentiation of certain cell lines or were due to other mechanisms. The effects of BUdR on proliferating and differentiating cells in the 12-day mouse embryo were therefore examined and special attention was given to the proliferating cells of the rhombic lip which give rise to the Purkinje cells. When the embryos were treated with BUdR the mitotic index of the neuroepithelium of the rhombic lip doubled in value 3 h after treatment and remained high until 24 h later. By using the colchicine index it was calculated that the mitotic duration in the BUdR-treated embryos lasted at least 2 h and that in the control embryos less than 1 h. When the cell generation time in the BUdR treated animals was calculated the length of the S-phase was increased by about 50%. It was thus concluded that BUdR caused an increase in the duration of the S-phase and mitosis, together making the cell cycle 5 h longer than normal. Eighteen hours after treatment many neuroepithelial cells became degenerative. By radioautography it was demonstrated that the degenerating cells were in their second DNA-synthetic phase following BUdR injection and that cells which incorporated BUdR and were differentiating into neurons were not affected. By injecting [3H]BUdR it was found that many cells which incorporated the analogue were able to leave the proliferative population after their first cell division. They migrated to the periphery where they developed into apparently normal Purkinje cells. The additive effects of cell death and retardation of the cell cycle caused a 15% deficit of Purkinje cells in the postnatal cerebellum but the BUdR did not interfere with their differentiation. Thus, contrary to the BUdR effect on cultures of embryonic cells, in vivo the drug causes cell death and a delay in the cell cycle time. Our experiments therefore seem to indicate that the congenital malformations caused by BUdR in the mammalian embryo are caused by cell death and growth retardation rather than by interference with the process of differentiation.