The effect of chronic administration of salmon calcitonin (SCT) to rats with adjuvant arthritis was studied. Compared to arthritic controls, 10 units/kg day in a divided dose of SCT given s.c. resulted in a significant inhibition of paw edema and a reduction in the marked deterioration of the bones of the hind limbs, particularly those of the tarsus, as visualized by x-ray. In addition, a pronounced increase in bone density in the proximal metaphyseal area of the tibia was consistently observed in calcitonin treated animals. Animals treated with SCT also showed improved clinical symptoms including increased mobility and decreased sensitivity to handling compared to arthritic controls.Combinations of SCT with each of several antiinflammatory agents, including phenylbutazone, hydrocortisone acetate and ACTH, produced greater degrees of inhibition of paw edema and ankle bone deterioration than did the individual agents alone. In addition, administration of calcium chloride solution, per os, alone and in combination with SCT partially inhibited the disease.