The participation of corticosteroids and alpha 2-macroglobulin (alpha 2-M) in counter-irritation (CI) induced by carrageenin was studied. We observed that CI could be produced when the second stimulus was done after 24 h, at which time glucocorticoids had already returned to their basal levels but alpha 2-M levels were maximum. When the level of alpha 2-M decreased (48 h) the counter-irritatory effect was not observed. As the synthesis of this protein is corticosteroid-dependent, we did not observe CI in adrenalectomized rats. However, when adrenalectomy was done 20 h after the first stimulus, allowing the synthesis of large amounts of alpha 2-M, the carrageenin stimulus again produced CI. Furthermore, adrenalectomized rats treated with aprotinin, a kallikrein inhibitor, after the first carrageenin stimulus, showed a reduction of the response to the second carrageenin stimulus. These observations demonstrated that corticosterone was not directly responsible for this inhibitory effect. Nevertheless, the anti-inflammatory factors involved in CI were, in fact, corticosteroid-dependent and alpha 2-M was probably one of the main factors.