In the present study, we assayed the antioxidant properties of Ginkgo biloba (Gb) extract on rats submitted to 21 d of chronic hypoxia. Doses of 25 and 50 mg/kg were examined. Oxygenated free radical production measured by the chemiluminescence technique was significantly decreased in treated rats compared to control rats placed in similar experimental conditions, and this effect was more significant at the 50 mg/kg dose. On the other hand, no antioxidant enzyme activities of the drug were observed towards red blood cells. These results suggest that ginkgo biloba extract has a free radical scavenging action. These antioxidant properties could explain the beneficial hematological properties of Gb extract.