Double strand breaks (DSBs) are highly damaging DNA lesions that can destabilise the genome and generate a suite of adverse physiological outcomes in the oocyte and early embryo. While it is therefore likely that these cells possess a sophisticated suite of protective mechanisms to ameliorate such damage, the precise nature of these defence systems are yet to be fully elucidated. To improve our understanding of these systems, the aim of this research project was to characterise the response of oocytes to etoposide, a chemotherapeutic agent with the ability to elicit DSBs. We demonstrated significant developmental changes in etoposide vulnerability, with fertilisation of the oocyte leading to a rapid enhancement of its cellular defence machinery. Using a parthenogenic model we showed that this response was mediated, at least in part, by permeability glycoprotein (PGP), an endogenous multidrug efflux transporter that is up-regulated, translocated to the oolemma and phosphorylated upon oocyte activation. Moreover, our evidence from dye exclusion assays conducted in the presence of a specific PGP pharmacological inhibitor (PSC833), illustrated that these events significantly increase (p<0.01) efflux activity across the zygote membrane, thereby enhancing the ability of these cells to exclude genotoxicants capable of eliciting DSB formation. Future studies will focus on the examining the fertilisability of the etoposide treated MII oocyte and its capacity to repair the damage inflicted by such insults. Specifically, we aim to define the activity of integral checkpoint mediators and the classical DSB repair pathways (homologues recombination and non-homologous end joining) to respond to and/or resolve DSB DNA damage to ensure successful embryogenesis.