Controlled ozone administration has been shown to promote an oxidative preconditioning or adaptation to oxidative stress by increasing endogenous antioxidant systems. In the present study, the effects of ozone administration either prophylactically or therapeutically on the alterations of oxidant status in adjuvant arthritic rats have been studied. Seven groups of rats were used: (1) normal control group; (2) control arthritic group (21 days); (3) prophylactic ozone group: arthritic rats received fifteen intra-rectal applications of ozone/oxygen