1. Recent studies demonstrated that inhibition or genetic inactivation of the enzyme poly (ADP-ribose) polymerase (PARP) is beneficial in myocardial reperfusion injury. PARP activation in the reperfused myocardium has been assumed, but not directly demonstrated. Furthermore, the issue whether pharmacological PARP inhibition affords long-term functional benefit in the reperfused myocardium has not been explored. These questions were addressed in the present study. 2. In a rat model of myocardial ischemia (1 h) and reperfusion (up to 24 h), there was a marked and significant activation of PARP in the ischemic borderzone, as determined by poly(ADP-ribose) (PAR) immunohistochemistry. PAR localized to the nuclei of myocytes and infiltrating mononuclear cells. In the core of the infarction, necrotic tissues and diffuse PAR staining were observed. PARP activation remained markedly detectable 24 h after reperfusion. The PARP inhibitor 3-aminobenzamide (20 mg kg(-1) intraperitoneally 10 min before reperfusion, and every 2 h thereafter for 6 h) markedly reduced the activation of the enzyme in myocytes. 3. 3-aminobenzamide significantly protected against myocardial morphological and functional alterations at 24 h post-reperfusion. Notably, infarct size was reduced, circulating creatine kinase activity was attenuated, and myocardial contractility (dP dt(-1)) was restored by 3-aminobenzamide. 4. Our results demonstrate a significant and prolonged activation of PARP in the reperfused myocardium, localizing to the necrotic area and the ischaemic borderzone. Furthermore, the studies demonstrate that PARP inhibition affords long-term beneficial morphological and functional effects in the reperfused myocardium. These data strengthen the notion that pharmacological PARP inhibition is a viable novel experimental approach for protection against myocardial reperfusion injury.