The anticonvulsant drug, Dilantin, in many cases must be taken by epileptic mothers to control seizures during pregnancy, but unfortunately, it has been characterized as a human teratogen. It has also been demonstrated that many of the teratogenic effects of Dilantin occur during postimplantation, but some studies implicate a detrimental role for Dilantin during the preimplantation stages of development. Some of the postimplantation effects include congenital malformations and the potential'loss of the fetus. Our lab has proposed that in preimplantation mouse embryos the drug may be altering the timing of expression of cell cycle regulatory proteins and therefore, we have begun to examine the expression of these proteins. Thus, it was the goal of this study to characterize and localize various cell cycle proteins at specific time points in normal in vivo preimplantation mouse embryos, as this will provide important baseline information for studies on how anticonvulsant drugs may alter cell cycle regulation in embryos.Western blotting has confirmed the presence of cyclin BI in G1 of the first cell cycle. Both cyclin E and CDK2 were not detected in GI or G2/M of the first cell cycle or GI of the second cell cycle.From the immunogold TEM experiments, the density of cyclin B1 staining was observed to be the highest at G1 of the first cell cycle and declined at S and G2/M. Cyclin B 1 was detected in all regions of the embryo including the microvilli, cortical cytoplasm, interior cytoplasm, and was observed to be associated with vesicles and some filaments. The gold particles at GI, S, and G2/N4 of the first cell cycle and G1 of the second cell cycle appear to be associated with filamentous and membraneous structures and not free in the cytoplasmic spaces. Cyclin B 1 expression was more concentrated around vesicles at G1 of the first cell cycle and in general, was more concentrated around vesicles than in microvilli and cortical cytoplasm, interior cytoplasm, or around filaments at each cell cycle stage tested. / Department of Biology