We have previously shown that the addition of Granulocyte-Macrophage Colony Stimulating Factor (GM-CSF) during in vitro oocyte maturation (IVM) can improve embryo development. The present study was undertaken to understand if the addition of GM-CSF during IVM improves mitochondria membrane potential of oocytes. C57Bl6 x CBA F1 female mice age 21-23 days were injected with 5IU of eCG and cumulus-oocyte complexes (COCs) aspirated from large antral follicles 46-48 h post-injection. Ten COCs were cultured per 50μL drop in bicarbonate-buffered α-MEM containing 3 mg/ml BSA,1 mg/ml Fetuin and 5 mIU/mL FSH plus 0 or 10ng/ml of GM-CSF. 16 hours later matured COCs and denuded oocytes were incubated for 15 minutes in HEPES buffered alpha-MEM media with JC-1 dye (5, 5_,6,6_-tetrachloro- 1,1_,3,3_ tetraethylbenzimidazolyl-carbocynanine iodide; Molecular Probes, OR, U.S.A.). After 15 minutes of incubation stained oocytes/COCs were washed to remove excess stain and then mounted on a glass slide for imaging. The mean red and green intensities were analysed in four different regions within three areas of oocyte: outer area, intermediate area, and peri-nuclear area. Fluorescence was visualised in Cell Voyager CV1000 Confocal Scanner (Yokogawa, Japan) and images were analysed using Fiji Image J software. The experiments were replicated three times with 45-60 COCs/oocytes per treatment group. Instrument settings were kept constant for each replicate. Data were analysed using a univariate general linear model in SPSS. The addition of GM-CSF during IVM had no effect on mitochondrial membrane potential in denuded oocytes, however, there was a significant increase in mitochondrial membrane potential in mature COCs (2.42 vs 1.92; P <0.01). In conclusion, we have shown that the addition of GM-CSF during IVM increases COC mitochondrial activity which may contribute to increase embryo development seen previously.