A mathematical model is developed whereby the longitudinal magnetization of phosphocreatine (PC), ATP, Pi, and total phosphate (PT) can be calculated on the basis of assumed chemical rate constants (kappa i) and spin lattice relaxation times of the muscle PC in equilibrium ATP in equilibrium Pi exchange system. By means of this model, some unexplained 31P nuclear magnetic resonance (NMR) spectroscopy results from the literature (e.g., a decrease of PT in a closed system) could be explained simply on the basis of the physiological variability of kappa i. Moreover, appropriate model simulations indicate that 1) 31P-NMR spectra obtained with short relaxation delays may be influenced to various extents by the metabolic and physicochemical status of the muscle; 2) the assessment of kappa i by standard NMR spectroscopy techniques may be extremely critical; 3) delta PC/delta Pi, as obtained from conventional 31P-NMR spectra, may represent a sensible index of kappa 2 (the pseudo first-order chemical exchange rate constant of the adenosinetriphosphatase reaction); 4) delta PC/delta Pi changes as detected from sequential (short relaxation delays) 31P-NMR spectra obtained in humans during metabolic transients (e.g., during transition from rest to work and vice versa) may represent an index of transient changes of kappa 2.