In vitro culture of mammalian preimplantation embryos is associated with various developmental disorders such as retardation in development and cell damage. The molecular mechanisms underlying these processes are poorly understood. One of the possible reasons may be the unphysiologically high oxygen concentration used for culture. Four-day-old rabbit blastocysts were cultured with 5% O2 (physiologic oxygen concentration) or 20% O2 (usually used for in vitro culture) for 4 hr. Differences in gene expression were analysed by differential display reverse-transcriptase polymerase chain reaction (DD RT-PCR). Thirty-two differentially expressed RNA bands were found. Two of them revealed a high sequence homology to the equine NADH-ubiquinone oxidoreductase chain 2 (ND2), a subunit of complex I of the respiratory chain. mRNA expression of ND2 was increased in blastocysts cultured with the higher oxygen concentration. Increased expression of ND2 was confirmed by semiquantitative and semiquantitative competitive RT-PCR.