Recent 31P-NMR saturation transfer measurements of flux between Pi and ATP in the perfused rat heart (Kingsley-Hickman, P., Sako, E.Y., Andreone, P.A., St. Cyr, J.A., Michurski, S., Foker, J.E., From, A.H.L., Petein, M. and Ugurbil, K. (1986) FEBS Lett. 198, 159-163) have given a P/O ratio (mols ATP synthesised/atoms oxygen consumed) which was close to 6. This anomalously high value was attributed to exchange in the reaction catalysed by the mitochondrial F1F0-ATP synthase. We show here that this exchange could also be catalysed by the glycolytic enzymes glyceraldehyde-3-phosphate dehydrogenase and phosphoglycerate kinase. 31P-NMR saturation transfer measurements of the exchange catalysed by these enzymes in vitro, under conditions designed to mimic those present in the perfused rat heart, have shown that they could catalyse a quantitatively significant Pi-ATP exchange in vivo. A three-site exchange model is used to investigate the effects of Pi-ATP exchange on saturation transfer measurements of the reverse flux in the creatine kinase reaction. A discrepancy in the measured and forward and reverse fluxes in this reaction has been attributed previously to the participation of the gamma-phosphate of ATP in other exchange reactions.