Arthritis was induced in 9-week-old female Dark Agouti rats by injecting type II collagen. Serum levels of the derivatives of reactive oxygen metabolites (dROMs), which are oxidative stress markers, and C-reactive protein (CRP) in arthritic rats that were exposed to a pressure of 1.25 atmospheres absolute and an oxygen concentration of 36% for 3 weeks (arthritis + HBO group) were compared to those of control rats (control group) and arthritic rats that were not exposed to hyperbaric oxygen (arthritis group). The body weights of the arthritis and arthritis + HBO groups were lower than that of the control group, whereas no difference in the body weight was observed between the arthritis and arthritis + HBO groups. The serum levels of dROMs and CRP in the arthritis group were higher than those in the control and arthritis + HBO groups. No difference in the serum level of CRP was observed between the control and arthritis + HBO groups. These results indicate that the conditions of hyperbaric oxygen exposure used in this study are effective for reducing the levels of reactive oxygen species, which are overproduced during arthritis.