The present study was designed to assess the anti-inflammatory activity of hyperbaric oxygen treatment by comparing it with that of diclofenac, a nonsteroidal anti-inflammatory drug, and also to investigate whether hyperbaric oxygen treatment enhances the anti-inflammatory effect of diclofenac in carrageenan-induced paw edema which is commonly employed as an acute inflammation model in rats. Hyperbaric oxygen treatment and diclofenac (20 mg/kg) markedly reduced the carrageenan-induced paw edema in rats. In other words, they displayed anti-inflammatory activity. On the other hand, hyperbaric oxygen treatment did not consistently modify the anti-inflammatory effect of diclofenac in this model. (C) 2001 Elsevier Science B.V. All rights reserved.