In vitro maturation (IVM) of oocytes obtained from medium sized antral follicles could avoid the need for superovulation for in vitro fertilisation. FSH is routinely added to culture media during IVM but the effects on oocyte health are unclear. The aim of this study was to assess the effect of FSH on 1) the metabolic activity of individual mouse cumulus-oocyte-complexes (COCs) matured in vitro,and 2) the genesis of chromosomal abnormalities. 274 mouse COCs were isolated from the antral follicles of 25-28 day old sexually immature B6CBF1 mice and individually cultured for 15-16 hours in 5 microlitre drops of Minimal Essential Media Eagle supplemented with pyruvate, bovine serum albumin and increasing concentrations of FSH (0, 2, 20, 200, 2000 nanograms per millilitre). Following culture, 3 microlitre aliquots of the experimental and control media were non-invasively assessed for pyruvate uptake or production, glucose uptake and lactate production. Substrate concentrations were measured fluorimetrically using a Cobas Bio centrifugal autoanalyser. After incubation, denuded oocytes were assessed for maturation status, spread, counterstained with DAPI and chromosome counts performed. Significantly more oocytes completed maturation in 2000 nanograms per millilitre FSH compared to controls (78.7% plus/minus SD 6.9 v. 60.7% plus/minus SD 6.7; p<0.05). There was a 2-3 fold increase in glucose uptake, from 22.8 plus/minus SD 7.2 in the absence of FSH to 52.1 plus/minus SD 9.9 picomoles/COC/hour in 2000 nanograms per millilitre FSH (p<0.001); and in lactate production from 33.3 plus/minus SD 5.7 in the absence of FSH to 76.0 plus/minus SD 8.0 picomoles/COC/hour in 2000 nanograms per millilitre FSH (p < 0.001). COCs took up pyruvate in the absence of FSH and produced it in the presence of FSH. Chromosomal abnormalities increased with increasing concentrations of FSH, with significantly more hyperploidies observed at the highest concentration of FSH (0% v. 15%; p<0.05). These results indicate that exposure to high levels of FSH results in altered COC metabolism, increased nuclear maturation and chromosomal abnormalities.