Recent studies from this laboratory [l, 2] on the reaction mechanism of the pentose phosphate pathway in rabbit and rat liver cells have accentuated the role of enzyme-catalyzed exchange reactions as a means by which tracer carbon is relocated in the absence of a net metabolic flux. While this is an intrinsic property of the group transferring enzymes of the pathway and is seen as a deleterious property of14C for the mapping and quantitation of metabolic pathways, it has re-emphasized the possible usefulness of isotope exchange reactions in understanding the mechanism of action of nature's catalyst.