During follicle growth swine granulosa cells are physiologically exposed to a progressive oxygen shortage. It has already been shown that hypoxia stimulates angiogenesis through an increase of VEGF production, however, despite considerable progress in the understanding of the final events induced by cellular hypoxia, the signal transduction pathway remains elusive. Recent evidence suggest a role for Reactive Oxygen Species (ROS) as hypoxia signal transducer. Granulosa cells were isolated from pig follicles (> 5 mm) and cultured for 18 h in normoxic (19% O2), hypoxic (5% O2) or anoxic (1% O2) conditions. Following the incubation ROS (O2- and H2O2) production and the activity of scavenging enzymes (SOD, catalase and peroxidase) were determined. It was apparent from our data that ROS generation was reduced by hypoxia. On the contrary, SOD and peroxidase, but not catalase, increased their activity. Further studies are needed to verify whether ROS are involved in signalling hypoxia.