The inhibitory action on rat liver phosphoglycerate kinase of structural analogs of the two substrates of this enzyme (beta,gamma- methyleneadenosine 5'-triphosphate for ATP and 2-phosphoglycerate for 3-phosphoglycerate) was studied. In the direction of ADP utilization, the inhibition patterns obtained with beta,gamma-methyleneadenosine 5'-triphosphate are compatible with the reaction mechanism proposed previously (Lavoinne, A., Marchand, J.C., Chédeville, A. & Matray, F. (1983) Biochimie 65, 211-220). In the direction of ATP utilization, the normally observed nonlinear kinetics were changed into linear kinetics in the presence of these substrate analogs. Our results suggest that saturation of the substrate sites induces a conformational change of the enzyme.