With microdialysis, we monitored cardiac interstitial fluid (ISF) levels of allopurinol, its metabolites, and the adenine nucleotide breakdown products (ANBP), inosine, hypoxanthine (HYP), xanthine (Xa), uric acid (UA) in dogs that received 1 and 10 mg/kg allopurinol intravenously (i.v.). Half-life (t1/2) of drug penetration into the heart was dose independent (1.8 min), whereas for the 10-mg/kg dose terminal elimination t1/2 (96 min) was much prolonged and ISF clearance (9.6 l/min kg) was reduced as compared with that induced by 1 mg/kg (28 min and 30.4 l/min kg) probably due to capacity limitation of allopurinol conversion to oxypurinol by Xa dehydrogenase/oxydase (Xa D/O). Inhibition of Xa D/O activity by allopurinol resulted in a dose-dependent increase in ISF HYP and Xa levels and a decrease in UA level. For a 10-mg/kg dose, maximal effect was attained approximately 40 min after drug injection. Allopurinol (1 mg/kg) given 30 min after the start of 40-min coronary artery occlusion during ischemia entered the ischemic zone ISF very slowly as compared with that of the control zone; the no-reflow phenomenon was evident because the levels became similar in both zones only 15 min after initiation of reperfusion. To examine cardioprotective efficiency, we administered allopurinol (10 mg/kg) 40 min before 40-min occlusion; it had little effect on total ANBP release during ischemia but facilitated washout of ANBP from the ischemic zone during reperfusion, thus manifesting protective efficacy against reperfusion injury and no-reflow. As shown by the lack of ischemia-induced increase in ISF Xa, myocardial Xa D/O activity was completely blocked by allopurinol.