The aims of the present study were to investigate the effects of exogenous peroxynitrite (ONOO-) on reperfusion arrhythmias in anaesthetized rats, and to detect endogenous and exogenous ONOO--induced chemiluminescence (CL) signals in rat whole blood, which was collected during baseline and in the first minute of reperfusion. ONOO- infusion in ischemia/reperfusion (I/R) applied groups caused significant decreases in mean arterial pressure (MAP) and heart rate (HR). Ventricular fibrillation (VF) incidences in the vehicle, ONOO-, and dec-ONOO- infused groups were 63, 100, and 20%, respectively. In control group CL signal was 136 +/- 34 mV during the resting period and was increased to 336 +/- 57 mV with reperfusion. Also, the effects of SOD + CAT, L-NAME and urate were investigated. Ventricular tachyeardia (VT) incidence was decreased significantly in SOD + CAT and urate; VF incidence was decreased significantly in SOD + CAT applied groups. CL signals were inhibited with SOD + CAT, L-NAME, and urate. Exogenous ONOO- infusion during I/R was also investigated. CL signal in exogenously ONOO- infused group is increased 423 % during reperfusion. Only urate infused group VF incidence was decreased significantly. CL signals of ONOO- infused groups were inhibited by SOD + CAT, L-NAME, and urate. Based on the results of the current study, ONOO- seems to be one of the key mediators of reperfusion arrhythmias in anaesthetized rats. (C) 2003 Elsevier Ltd. All rights reserved.