Morphological studies of the heart muscle were carried out in healthy rats as well as treated with hydralazine or binazine after administration of adrenaline, noradrenaline, isoprenaline, regitine or propranolol. Hydrazinophtalazines (25 mg/kg body wt, daily) were given during 7 months. After discontinuation of this treatment the drugs were administered during 15 days. Evident changes (e.g. ... [Show full abstract] inflammatory infiltrations, focal necrosis and fibrosis) were found after administration of noradrenaline, adrenaline or isoprenaline. No differences were found in the reactivity to catecholamines in healthy and hydrazinophtalazine-treated rats.