Inadequate oxygenation of cardiac muscle leads to rapid loss of high energy compounds essential for contractile function. ATP can be regenerated by synthesis de novo, a route operating at a relatively slow rate in the heart. Myocytes isolated from mature rat heart have been used to measure the rate of ATP synthesis de novo from both [14C]glycine and [14C]ribose. Incorporation of glycine into ATP is accelerated 10-fold in the presence of 1 mM ribose. Myocytes also accumulate both precursors into IMP and four other metabolites on the de novo synthesis pathway. These metabolites represent 80% of the glycine entering the pathway. The potential of de novo synthesis for restoration of adenine nucleotides appears to be limited by the rates of early reactions, adenylosuccinate synthetase being only one of the enzymes operating at a sufficiently slow rate to make this pathway an inherently weak route for the restoration of normal energy status in post-ischemic myocardium. Interventions are being sought to alleviate these apparent metabolic delays.