Glucocorticoid therapy induces rapid involution of chronic granulomatous inflammation which has been provoked by injecting carrageenin solution subcutaneously in rats. Cortisol acetate injected into the granuloma pouch at a dose of 3 mg/kg/day for 3 days caused a marked involution of the granulomatous inflammation. After withdrawal of the steroid treatments, however, rebound of the inflammation took place resulting in a rapid recovery not only in the wet weight and the dry weight of the granuloma but also in vascular permeability response and the volume of the exudate. Determination of vascular permeability of the granulomatous tissue were performed with the aid of radioiodinated human serum albumin which was injected intravenously 30 min before sacrifice. Vascular permeability was expressed in terms of radioactivity exuding into the granuloma pouch fluid for the 30 min. By repeated administrations at 12 hr intervals with hydroxyurea (250 mg/kg×6, injected into the granuloma pouch) after the withdrawal of the steroid treatments, rebound of the inflammation not only in proliferative phases but also in exudative phases was suppressed almost completely.