The presence of a divalent metal ion together with a catalytic amount of inosine 5'-diphosphate (IDP) is essential for the formation of pyruvate from oxalacetate catalyzed by purified rat liver cytosol phosphoenolpyruvate carboxykinase (PEPCK). With decreasing order of effectiveness, this pyruvate-forming activity was supported by micromolar levels of Cd2+, Zn2+, Mn2+, and Co2+. At the same concentrations, Mg2+ or Ca2+ was not effective. Combinations of Cd2+ with either Zn2+, Mn2+ or Co2+ were not additive with respect to the pyruvate-forming activity of PEPCK. Kinetic determination, with Cd2+ as the supporting cation, showed a 1:1 stoichiometry of interaction between each enzyme molecule and the nonconsumable substrate IDP. With 10 muM added Cd2+, the apparent Km for oxalacetate was 41 muM, and the apparent Ka for IDP was 0.25 muM. With Zn2+ or Mn2+, the apparent Ka for IDP was 0.2 or 0.13 muM, respectively. The effect of divalent transition-metal ions on PEPCK-catalyzed formation of phosphoenolpyruvate from oxalacetate was also investigated. Under steady-state conditions, the basal activity with MgITP was effectively enhanced with micromolar levels of Mn2+, Cd2+, or Co2+ included in the assay. The Vm increased 7- and 3.6-fold, and the apparent Km for MgITP changed by about a factor of 2 with the optimal concentrations of Mn2+ and Co2+, respectively. The most striking changes were in the apparent Km values for oxalacetate, which decreased to one-third and one-tenth when either Mn2+ or Co2+ was present in the assay together with Mg2+. The possible physiological importance of this kinetic effect is discussed.