31P-NMR spectroscopy has become the major tool for studying myocardial high energy phosphates. Conflicting results concerning NMR visibility of ATP in ischaemic myocardium were reported. A detailed study was undertaken to resolve this controversy. After cardioplegic arrest, canine hearts were excised and preserved for 24 h at 1 degree C (group 1) or for 6h at 23 degrees C (group 2). ATP breakdown was followed by 31P-NMR spectroscopy in a transmural piece of the anterior wall introduced in the NMR magnet, and by HPLC analysis using serial transmural biopsies from the rest of the anterior wall. At both temperatures, identical relative ATP decay curves were obtained, whether measured by NMR or by HPLC. Absolute quantification of ATP was carried out after varying periods of ischaemia at 1 degree C. The NMR-measured ATP concentration was 106 +/- 8% of the ATP concentration determined by HPLC. From our experiments, we conclude that ATP visibility for 31P-NMR spectroscopy is complete and constant during prolonged periods of hypothermic ischaemia in canine hearts.