The cardiotoxic effect of the beta-adrenergic agonist isoproterenol was studied in cultured neonatal rat myocytes. A progressive increase in irreversible cell injury as determined by leakage of the cytoplasmic enzyme alpha-hydroxybutyrate dehydrogenase (alpha-HBDH) from the cells was noted at concentrations above 2.5 x 10(-4) M isoproterenol (exposure time 6 h). The isoproterenol-induced cell damage was reduced or prevented by several free radical scavengers: the application of Trolox C, a water-soluble vitamin E analogue, ICRF-159, a chelator of divalent cations, ascorbic acid, a potent antioxidant, as well as the enzymatic free radical scavengers superoxide dismutase and catalase reduced alpha-HBDH release. This study corroborates the hypothesis that oxidation products of isoproterenol, especially the formation of oxygen- and/or oxygen-derived free radicals, are responsible for the cytotoxicity observed after prolonged exposure to isoproterenol. In contrast to isoproterenol, exposure to 5 x 10(-4) M fenoterol, another beta-adrenergic agonist which is not oxidized, does not impair the viability of the myocytes. Moreover, application of the beta-blocker propranolol (10(-4) M, 10(-5)M) in combination with 5 x 10(-4) M isoproterenol does not prevent alpha-HBDH release. These findings suggest that isoproterenol-induced cardiotoxicity is not the result of excessive beta-adrenoceptor activation, but is mediated by the formation of free radicals.