The persistence and effects of 3H-labelled and intra-articularly administered 11 beta-hydroxy-16 alpha,17 alpha,21-trimethylpregna-1,4-diene-3,20-dione (rimexolone, [3H]-Org 6216 have been investigated in a model of fibrin-induced mono-articular arthritis in the rabbit. It appears that joint swelling is suppressed in a dose-dependent way when the drug is administered into the arthritic joint. With a dose of 15 mg the joint circumference was restored to the initial value recorded prior to antigen challenge within 14 days. Administration of rimexolone into the non-arthritic joint also suppressed contralateral arthritic joint swelling, but this effect was less pronounced and shorter lasting. Radioactivity from [3H]-rimexolone at doses of 5 or 15 mg disappears slowly from both non-arthritic and arthritic knee joints.