[en] Information about enzyme systems can be obtained from studies of isotope exchange reactions. NMR can monitor these reactions continuously, even in intact biological samples. In this paper ¹H NMR is used to study a ¹H/²H exchange process which involves multiple labelling of NADH. The NADH is an intermediate in an exchange process involving the glycolytic enzymes glyceraldehyde phosphate dehydrogenase, lactate dehydrogenase, triose phosphate isomerase and aldolase. The form of the observed exchange is predicted by an extended theory of isotope exchange. (author)