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The effect of cyclic-AMP-dependent phosphorylation on the activity of isolated pig liver pyruvate kinase was studied. It was found that the major kinetic effect of the phosphorylation was to reduce the affinity for the substrate phosphoenolpyruvate, ,5 for this substrate increasing from 0.3 to 0.9 mM upon phosphorylation. The cooperative effect with phosphoenolpyruvate was enhanced, the Hill constant nH increasing concomitantly from 1.1 to 1.5. V was unaltered. The change in activity occurred in parallel with the phosphate incorporation, except during the initial part of the reaction, when inactivation was correspondingly slower. The affinity for the second substrate ADP was unchanged, with an apparent K,,, of 0.3 mM at saturating concentration of phosphoenolpyruvate. Likewise, the requirement for potassium was unaffected, whereas the phosphoenzyme required a higher concentration of magnesium ions for maximal activity, compared with the control enzyme. The inhibitory effect of the phosphorylation was counteracted by positive effectors, fructose 1,6-bisphosphate in micromolar concentrations completely activated the phosphoenzyme, resulting in an enzyme with properties similar to the fructose I ,6-bisphosphateactivated unphosphorylated enzyme, with KO ,5 for phosphoenolpyruvate about 0.025 mM and with a Hill constant of 1 .l. Hydrogen ions were also effective in activating the phosphoenzyme. Thus, when pH was lowered from 8 to 6.5 the inhibition due to phosphorylation was abolished. The phosphoenzyme was sensitive to further inhibition by negative effectors such as ATP and alanine. 2 mM ATP increased I& for phosphoenolpyruvate to 1.5mM and nH to 2.3. The corresponding values with alanine were 1.3 mM and 3.9. Phosphorylation is thought to be an additional mechanism of inhibition of the enzyme under gluconeogenetic conditions.