The rate constant of the -adenosine triphosphate to -adenosine diphosphate conversion was measured using 31 P nuclear magnetic resonance magnetization transfer in resting and contracting in vivo rat skeletal muscle. Theoretically, the rate constant should be the sum of the rate constants of the reactions catalyzing ATP-ADP exchange. In resting muscle, the conversion rate constant was 0.4 s Ϫ 1 and -ATP intrinsic T 1 was 1.7 s. The velocity of conversion was 3.8 mM s Ϫ 1 . During stimulation, phosphocreatine fell to 36% and ATP to 82% of initial values. The rate constant and velocity of -phosphoryl conversion increased to 0.8 s Ϫ 1 and 6.3 mM s Ϫ 1 , respectively, but did not reach expected levels, i.e. the product of the ATP concentration with the sum of pseudo first-rate constants of the individual reactions. These conversion velocities should be higher than reverse creatine kinase velocities, previously measured to be 10 mM s Ϫ 1 in resting muscle and 7.5 mM s Ϫ 1 in contacting muscle and confirmed in this work. The discrepancy between expected and observed data could be due either to compartmentation of part of the -ATP in pools exchanging slowly with the bulk of cellular ATP, or to ADP binding to macromolecules thus preventing full ADP saturation during magnetization transfer.