The simultaneous production of nitric oxide (NO) and superoxide leads to the formation of a potent toxic metabolite peroxynitrite (ONOO-). However, ONOO- at low concentrations has been found to exert cardioprotective effects. The purpose of the present study was to investigate the effects of exogenous ONOO- on ischaemia-reperfusion arrhythmias. We studied the concentration-response effects of ONOO- (0.4, 4, 40 mu M ml(-1) min(-1) for 20 min) in rat isolated hearts perfused with Krebs-Henseleit solution. The 0.4 mu M concentration of ONOO- was selected for further experiments since it did not affect the sinus rhythm. In the hearts subjected to 10 min of ischaemia followed by 10 min of reperfusion during 0.4 mu M ml(-1) min(-1) ONOO- infusion, the incidence of ventricular fibrillation was decreased significantly from 93% to 38% (n = 8) and none of the hearts had an irreversible ventricular fibrillation. Urate, a ONOO- scavenger (at 1 mM, n = 7), added to the perfusate 5 min prior to the coronary artery occlusion and maintained throughout the experimental period, did not significantly modify the beneficial effects of ONOO-. Although L-N-G-nitroarginine methylester (L-NAME) (100 mu M, n = 8) had no effect, superoxide dismutase (10 U ml(-1)) + catalase (100 U ml(-1)) increased the number of ventricular ectopic beats from 91 +/- 32 to 286 +/- 83 (n = 5) and augmented the incidence of irreversible ventricular fibrillation from 0% to 60%. There were no marked changes in the time of onset of the first arrhythmias in any group. These results suggest that ONOO- at a low concentration may exert beneficial effects on ischaemia-reperfusion-induced arrhythmias in rat isolated hearts. (C) 1999 Elsevier Science B.V. All rights reserved.