Up-regulation of the inducible isoform of nitric oxide synthase (iNOS) was determined during the development of adjuvant-induced arthritis in the rat. iNOS enzymatic activity, measured in spleen tissue, appeared and increased coincidentally with the appearance and degree of paw swelling and joint destruction in this arthritis model, when measured on days 0 through 21 subsequent to inoculation of the rats with adjuvant. The increase in enzymatic activity was paralleled by an increase in the plasma nitrite/nitrate (NOx) level and the appearance of immunoreactive iNOS, as measured by Western immunoblot, in the spleens of these rats. Prophylactic administration of N-iminoethyl-L-lysine (L-NIL) completely abolished iNOS activity (plasma NOx elevation) and effectively reduced both the swelling and radiographic changes in the joint tissues of the noninjected paw measured on day 21. However, therapeutic administration of L-NIL beginning on day 14 had no effect on the inflammatory or arthritic changes measured on day 21, even though plasma NOx levels were reduced to that of the naive controls. These results suggest that iNOS may be involved with the initial stages of the immune response to adjuvant injection, but its product, NO, does not mediate the chronic inflammation and joint destruction which occur during the later phase in this model.