AbstractThe effect of anaerobic metabolic changes on the flux of the creatine kinase reaction in bullfrog skeletal muscle was evaluated by31P saturation transfer NMR. During an anaerobic time course the reduction ratio (M+/M0) of the phosphocreatine signal and the spin‐lattice relaxation time measured in the presence of saturating irradiation at the ATP γ‐phosphate resonance were decreased; however, the intrinsic spin‐lattice relaxation time of phosphocreatine was constant (4 s). The pseudo‐first‐order rate constant for the forward reaction of creatine kinase (i.e., phosphocreatine to ATP) was increased during the time course, accompanied by a decrease in phosphocreatine concentration and an intracellular acidification. However, the flux was constant (4 mmol . kg−1wet wt . s−1) over the anaerobic time course. The fluxes analysed in several locations along the fibers, which were simultaneously measured by means of the one‐dimensional chemical shift imaging technique, were almost the same along the entire length of the fibers. These results indicate that the creatine kinase reaction is at equilibrium along the entire length of the fr