On the basis of the alternatives of direct inter-enzyme transfer vs. dissociation followed by random diffusion, two kinetic models for metabolite transfer between consecutive enzymes are developed. These two models are readily distinguishable experimentally for the transfer of 1,3-diphosphoglycerate (1,3-P2G) between glyceraldehyde-3-phosphate dehydrogenase (GPDH) and 3-phosphoglycerate kinase (PGK). Since 1,3-P2G is exceedingly tightly bound to PGK, the kinetics of its transfer to GPDH are predictably different for each of these two models. Our experiments unambiguously demonstrate that 1,3-P2G is directly transferred between these two enzymes via an enzyme-substrate-enzyme complex. This direct transfer is described by a Michaelis-Menten scheme in which PGK . 1,3-P2G is the "substrate" for GPDH. At high concentrations of PGK . 1,3-P2G, the transfer reaction becomes nearly PGK . 1,3-P2G concentration independent. The rate of the transfer reaction is activated 3.5-fold by saturating quantities of ATP and 20-fold by saturating quantities of 3-PG. Evidence is presented that the PGK . 1,3-P2G complex is structurally distinct from either PGK itself or other PGK . ligand complexes.