Full-grown blastocysts were cultured for 70-92 h in Dulbecco's modified Eagle's medium supplemented with fetal calf serum. They were treated with (1) aphidicolin, a specific inhibitor of eucaryotic DNA polymerase alpha, (2) dideoxythymidine (d2Thd), the precursor of dideoxythymidine triphosphate (d2TTP), a specific inhibitor of DNA polymerase beta and (3) d2TTP itself. Cytophotometric measurements of both the DNA content and the nuclear areas were performed. The results show firstly that trophectoderm differentiation into primary giant trophoblast cells is already irreversibly programmed at the blastocyst stage and does not depend on any DNA replication cycle from that stage onwards. Secondly, the typical enlargement of the trophoblastic nuclei, which occurs at the peri-implantation period, is not related to a proportional increase in DNA content. Thirdly, the onset and progress of DNA endoreduplication in trophoblastic cells is carried out by DNA polymerase alpha during the first part of gestation instead of DNA polymerase beta, as has previously been shown to be the case for mid-gestation rat trophoblast cells.