Utilizing a sensitive reverse transcriptase-polymerase chain reaction (RT-PCR) assay system, the time course of mRIMA expression of two transcription regulators, OCT4 and OCT6, was assessed in individual preimplantation human embryos. Examination of ova with three pronuclei and 1-cell through blastocyst stage embryos revealed that OCT4 mRNA was continuously expressed between the time of fertilization and 10+ cell stages, whereas OCT6 mRNA expression was not observed until the 10+ cell stage. The difference in the time of expression of OCT4 mRNA and OCT6 mRNA indicates that the two genes play differential roles in human embryogenesis. Nucleotide sequence homology for OCT4 and OCT6 among mammalian organisms supports the concept that genetic elements determining developmental events during embryogenesis are conserved in evolution.