Glutathione (GSH) is synthesised during oocyte maturation and represents the oocyte's main non-enzymatic defence against oxidative stress. Inadequate defence against oxidative stress may be related to poor embryo quality and viability. In the present study, bovine oocytes were matured in vitro in the presence of GSH ethyl ester (GSH-OEt), a cell permeable GSH donor, and its effects on subsequent fertilisation and embryo development were assessed. GSH-OEt significantly increased the GSH content of IVM oocytes without affecting fertilisation or Day 3 cleavage rates. Maturation in the presence of GSH-OEt did not significantly increase the blastocyst rate compared with control oocytes. However, 5 mM GSH-OEt treatment resulted in significantly higher blastocyst total cell number. The GSH level of IVM oocytes was significantly decreased in the absence of cumulus cells and when cumulus-oocyte complexes were cultured in the presence of buthionine sulfoximine (BSO), an inhibitor of GSH synthesis. The addition of GSH-OEt to cumulus-denuded or BSO-treated oocytes increased the GSH content of bovine oocytes and restored the rate of normal fertilisation, but not embryo development, to levels seen in control oocytes. Thus, GSH-OEt represents a novel approach for effective in vitro elevation of bovine oocyte GSH and improvement in blastocyst cell number.