We have developed 31P NMR spectroscopic methods to determine quantitatively relative levels of phosphorous-containing metabolites in the human myocardium. We have used localization techniques based on the rotating-frame imaging experiment and carried out with a double-surface coil probe. Information is obtained from selected slices by rotating-frame depth selection and from a complete one-dimensional spectroscopic image using phase-modulated rotating-frame imaging. The methods collect biochemical information from metabolites in human heart, and we use the fact that the phosphocreatine/ATP molar ratio in skeletal muscle at rest is higher than that in working heart to demonstrate that localization has been achieved for each investigation. The phosphocreatine/ATP molar ratio in normal human heart has been measured as 1.55 +/- 0.20 (mean +/- SD) (3.5-sec interpulse delay) in six subjects using depth selection and as 1.53 +/- 0.25 (mean +/- SD) in four subjects using spectroscopic imaging. Measurement of this ratio is expected to give a useful and reproducible index of myocardial energetics in normal and pathological states.