The enhancement of ATP regeneration following global myocardial ischemia in dogs by both ATP catabolic enzyme blockade and precursor infusion was investigated. The breakdown of AMP to adenosine is catalyzed by 5′-nucleotidase and this enzyme was inhibited during the ischemic period with either concanavalin A (Con A, 3 mg/kg) or α,β-methyleneadenosine 5′-diphosphate (AMP-CP, 250 μM). To provide additional ATP precursors, adenine (30 mg/kg) and ribose (25 mg/kg) (A/R) were also infused into the coronary vasculature during ischemia and recovery on cardiopulmonary bypass. Left ventricular myocardial ATP levels in control animals decreased to 52% of preischemic values during aortic cross clamping, but ATP levels in dogs treated with AMP-CP + A/R fell to only 67% of preischemic values (P < 0.05). During reperfusion, ATP levels in Con A + A/R (3.43 ± 0.26 μmole/g wet wt) and AMP-CP + A/R (3.77 ± 0.42) treated animals were higher than values found in control dogs (2.73 ± 0.16, P < 0.05). Infusions of A/R alone without enzyme inhibition did not increase ATP regeneration. The adenine nucleotide energy charge ratio was also increased by enzyme blockade with either inhibitor when combined with precursor infusion. On bypass, left ventricular myocardial blood flow (measured by the microsphere technique) was increased by 140% (P < 0.01) over control values in all groups receiving A/R; therefore, enhanced ATP levels were not merely the result of increased flow. Renal blood flow was not adversely affected by this combination of drugs as has been previously found with adenosine infusion and inhibition of adenosine catabolism.