Pyruvate carboxylase, Pyruvate:CO2 Ligase (ADP); EC 6.4.1.1, free of contaminating enzymatic activity and suitable for kinetic studies, was purified from rat liver. Protomers of the enzyme, rather than tetramers, were purified using agarose bead columns and the protomer molecular weight was found to be about 13,. CrATP and bidentate and monodentate CrADP were prepared and used along with other Cr-nucleotide complexes, which are substrate analogs of MgATP, as dead end inhibitors of pyruvate carboxylase. The Cr-nucleotides are effective initial inhibitors of pyruvate carboxylase but the inhibition becomes more severe with time. Finally, this tightening of the inhibition ceases and a linear time course is observed. Maximum inhibition of the enzyme occurs when it is incubated with the Cr-nucleotide in the presence of Mg and HCO^ prior to the initiation of the reaction. CrATP is a competitive inhibitor with respect to MgATP and a non-competitive inhibitor with respect to HCO-Bidentate and monodentate CrADP are both better inhibitors than CrATP and bidentate CrADP is a stronger inhibitor than monodentate CrADP. The amount of inhibition caused by various Cr-nucleotides may indicate the relative importance of the and "if* phosphate groups of MgATP in binding to pyruvate carboxylase. The results of the inhibition experiments are in agreement with the proposed mechanism of action of the enzyme which involves two partial reactions at separate catalytic sites, Possible explanations of the tightening of the inhibition phenomenon are discussed.