Phosphorus-31 nuclear magnetic resonance (NMR) spectra were recorded from freshly excised and resting anterior byssus retractor muscles of the mussel Mytilus edulis. The absolute concentrations of phosphometabolites measured by NMR compare well with the values obtained by a biochemical method. Quantitative measurements were achieved in several minutes by applying to the observed NMR signals a correction taking the saturation effects into account. The time evolution of the phosphometabolite concentrations reveals that the hydrolysis of phosphoarginine is a first-order reaction producing inorganic phosphate, whilst the adenosine triphosphate level remains constant for more than 10 h. The phosphoarginine hydrolysis rate varies as a function of the season and reaches a maximum during the reproduction period of the mussel. This increase in phosphoarginine consumption could be bound up with the higher excitability of the muscles during spring. The activation energy of the phosphoarginine hydrolysis reaction also depends on the season and the difference between the value determined in winter and that measured in spring is explained by a modification of the mode of action or of the proportions of the enzymes involved in the muscular metabolism.