THERE are serious limitations to techniques used to correlate biochemical parameters with function in metabolically active tissues. For the perfused heart such techniques include measurement of surface NADH fluorescence², comparison of coronary arterial–venous composition, and biochemical analysis of intracellular metabolites from biopsy specimens. Although the latter approach has yielded critical information, it is limited because local injury prevents accurate interpretation of mechanical function. Recently ³¹P nuclear magnetic resonance (NMR) has been used to analyse excised muscles and their acid extracts for intracellular concentrations of the sugar phosphates, glycolytic intermediates, free orthophosphates, phosphocreatine, ADP and ATP. In addition, the pH-dependent chemical shift of the orthophosphate resonance has been titrated² and used to determine the intracellular pH of intact muscles³. These muscles were neither perfused nor functioning, but we report here the assessment by ³¹P NMR of the myocardial energy status in the working, perfused rat heart subjected to various degrees of ischaemia.