At the time of fertilisation, oocyte activation initiates the upregulation of DNA repair pathways and cellular protective mechanisms to ensure that the fertilised cell is equipped to cope with the challenges of embryogenesis and later development. However, double strand breaks (DSB) within the oocyte DNA have been shown to interfere with the ability of the female gamete to generate a viable embryo, and are directly correlated with preimplantation errors, spontaneous abortion, and developmental abnormalities. The aim of this research was to characterise the intracellular mechanisms that protect oocytes against DSB-inducing molecules during the pre- (metaphase II, MII) and post-fertilisation (zygote and strontium activated parthenotes) phases of development. Immunocytochemical analyses confirmed that the common genotoxic agent etoposide, was capable of inducing potent DNA DSB damage in mouse MII oocytes (p<0.0001). Interestingly however, the sensitivity of oocytes to etoposide was significantly decreased following fertilisation/spontaneous activation (p<0.01); highlighting substantial differences in DNA damage sensitivity in maturing oocytes. This reduced sensitivity appeared to be attributed, at least in part, to an upregulation of multidrug resistant (MDR) efflux transport activity following oocyte activation/fertilisation. In this context, we demonstrated a rapid increase in the expression of the MDR, permeability glycoprotein (PGP), and its translocation to the oolemma following oocyte activation/fertilisation. The enhanced activity of PGP in activated/fertilised oocytes was demonstrated through dye exclusion assays in the presence and absence of the selective PGP inhibitor, PSC833. These data confirmed that PGP afforded the activated/fertilised oocytes protection against etoposide, with its inhibition resulting in the induction of comparable levels of DNA DSBs in all oocyte populations examined. Overall, this study demonstrated an intriguing difference in DNA damage sensitivity between pre- and post-fertilisation oocytes. Ongoing work will focus on assessment of the specific role of PGP and other MDR transporters in protecting the fertilised oocyte from DNA damage.