Arthritis induced in rats by mycobacterial adjuvant has been used for the study of compounds of known value in the treatment of rheumatoid arthritis in man. The development of the arthritic syndrome in treated and control rats was followed by measuring the changes in foot thickness of both hind-feet with a micrometer. This method allowed the effect of anti-inflammatory compounds to be expressed quantitatively. Anti-inflammatory activity was readily observed in certain steroids, pyrazolidines, salicylates and sodium aurothiomalate. Chloroquine and hydroxychloroquine were inactive. The inhibition obtained by daily treatment with the steroid paramethasone disappeared when treatment was withdrawn.