The effects of hyperbaric oxygen (HBO) during tissue preservation on flap survival have been investigated in free flaps in rats. Groin skin flaps were harvested, stored in either room air or HBO (100% oxygen at 2.9 atm absolute) at 23°C for 18 hours, and transplanted to the contralateral groin. Free flaps exhibit a high incidence of complete necrosis in the room air control. The survival of free flaps stored under HBO increased from 10% to 60% (p<0.05) after 18 hours of preservation. Skin flaps exhibited an increase in tissue hypoxanthine by 3.6-fold normal after 18 hours of storage in room air. HBO preservation prevented the accumulation of hypoxanthine and inhibited xanthine oxidase. Inhibition of the xanthine oxidase system may be one of the mechanisms of improved success of skin flap transplantation.