Reactivation of the pea mitochondrial pyruvate dehydrogenase complex was the result of dephosphorylation catalyzed by phospho-pyruvate dehydrogenase-phosphatase, an intrinsic component of the complex. Phosphatase activity was dependent upon divalent metal ions, with Mg(2+) more effective than Mn(2+) or Co(2+). The Michaelis constants for Mg(2+), Mn(2+), and Co(2+) were 3.8, 1.7, and 1.4 millimolar, respectively. Neither the rate nor the extent of activation of the phosphatase by Mg(2+) or Mn(2+) was effected by up to 100 units per assay of megamodulin. Calcium ions did not activate pea mitochondrial phospho-pyruvate dehydrogenase-phosphatase, and low concentrations of Ca(2+) antagonized activation by other divalent cations. Phosphatase activity was inhibited by fluoride and ortho-phosphate but not by molybdate or vanadate. Krebs cycle intermediates, adenylates, polyamines, amino acids, and phosphoamino acids were without effect upon pea mitochondrial phospho-pyruvate dehydrogenase-phosphatase activity in vitro.