Little is known about the repertoire of genes expressed following zygotic gene activation, which occurs during the two-cell stage in the mouse. As an initial attempt to isolate novel genes, we used previously prepared two-cell and two-cell subtraction cDNA libraries (Rothstein et al., Genes Dev 6:1190-1201, 1992) to isolate a panel of seven cDNA clones. Three cDNAs had no match in the current DNA sequence data banks and three others revealed sequence homology to portions of sequences in the data banks. One cDNA was 90% homologous to the ras-related gene Krev/rap 1A. The temporal patterns of expression of these genes during oocyte maturation and preimplantation development were analyzed by a reverse transcription-polymerase chain reaction (RT-PCR) assay developed to measure relative levels of mRNAs. Three distinct temporal patterns of expression, designated Classes 1-3, were found. The two Class 1 genes displayed an actin-like pattern, with a gradual decline in expression during oocyte maturation and through the two-cell stage, followed by increases at the eight-cell and/or blastocyst stages. The four genes in Class 2 were expressed at relatively high levels during oocyte maturation and through the one-cell stage and then declined abruptly between the one- and two-cell stages; an increase then occurred at the eight-cell and/or blastocyst stages. The expression of the gene in Class 3 declined during oocyte maturation, but then showed a transient increase at the one-cell stage, with only a very slight increase in synthesis at either the eight-cell or blastocyst stage.