The forward and reverse rates through the creatine-kinase (CK) catalyzed reaction, phosphocreatine + ADP + H+ creatine + ATP in the in vivo monkey brain were measured using the techniques of saturation transfer (ST) and inversion transfer (IT) 31P nuclear magnetic resonance (NMR) spectroscopy. Independent checks on the ST apparent longitudinal relaxation (τ) data could be obtained from the parameters determined from the IT analyses. At near-equilibrium it is assumed that the forward-to-reverse flux ratio lies close to 1.0. In the monkey brain the value for the forward-to-reverse flux ratio obtained is 1.37 ± 0.26 calculated from ST with average τ values from IT initial slopes, a value which is not significantly different from unity. The present NMR data point to the CK reaction in the living monkey brain being maintained at or near equilibrium
