Experiments were conducted on 18 dogs using an in situ blood-perfused canine heart model. Intracoronary infusion of AMP resulted in increased ATP and total adenine nucleotide levels. On reperfusion following a 15-min period of ischemia, ATP and total adenine nucleotide levels were significantly higher than control. Most important, contractile function recovered more rapidly in the AMP-treated dogs. It is therefore concluded that the delayed functional recovery noted after periods of ischemia is likely to be a direct result of delayed ATP resynthesis.