Adjuvant arthritis was induced in female Fisher rats by injecting their right hind paw with 0.1 ml Freund's complete adjuvant. The development of adjuvant arthritis was inhibited by hypophysectomy and by daily treatment of intact animals with the dopaminergic agent bromocriptine. Adjuvant arthritis developed normally if hypophysectomized or bromocriptine-suppressed animals were treated with either prolactin or growth hormone. Additional treatment with adrenocorticotropic hormone inhibited this restoration. Treatment of hypophysectomized rats with follicle-stimulating hormone, luteinizing hormone, and thyroidstimulating hormone had no effect. These results indicate that prolactin and/or growth hormone are necessary for the development of adjuvant arthritis, whereas adrenocorticotropic hormone has an inhibitory effect.