A growing body of evidence indicates that the pro-oxidant/anti-oxidant balance inside the ovarian follicle plays an important role in folliculogenesis. Therefore, the aim of the present study was to assess the redox status of follicular fluids collected from different-sized swine follicles. We quantified the most important reactive oxygen species (ROS), namely superoxide anion (O₂^–), hydrogen peroxide and hydroperoxides (ROOH); in addition, we examined the activity of the detoxifying enzymes superoxide dismutase, catalase (CAT) and glutathione peroxidase and the total non-enzymatic antioxidant capacity as determined by the ferric-reducing anti-oxidant power assay. Our data demonstrate that oxidative stress does not affect follicle growth because O₂^– levels do not change during follicle development, whereas concentrations of H₂O₂ and ROOH are reduced (P < 0.05). Surprisingly, all non-enzymatic and enzymatic scavengers examined in the present study, except for CAT, demonstrated reduced activity during follicle development (P < 0.05). Taken together, these results suggest that other factors could be involved in ROS detoxification during follicle development.