Time courses for the autooxidation of native and mutant sperm whale and pig myoglobins were measured at 37 degrees C in the presence of catalase and superoxide dismutase. In sperm whale myoglobin, His64(E7) was replaced with Gln, Gly, Ala, Val, Thr, Leu, and Phe; Val68(E11) was replaced with Ala, Ile, Leu, and Phe; Leu29(B10) was replaced with Ala, Val, and Phe. In pig myoglobin, His64(E7) was replaced with Val; Val68(E11) was replaced with Thr and Ser; Thr67(E10) was replaced with Ala, Val, Glu, and Arg; Lys45(CD3) was replaced with Ser, Glu, His, and Arg. The observed pseudo-first order rate constants varied over 4 orders of magnitude, from 58 h-1 (H64A) to 0.055 h-1 (native) to 0.005 h-1 (L29F) at 37 degrees C, pH 7, in air. The dependences of the observed autooxidation rate constant on oxygen concentration and pH were measured for native and selected mutant myoglobins. In the native proteins and in most mutants still possessing the distal histidine, autooxidation occurs through a combination of two mechanisms. At high [O2], direct dissociation of the neutral superoxide radical (HO2) from oxymyoglobin dominates, and this process is accelerated by decreasing pH. At low [O2], autooxidation occurs by a bimolecular reaction between molecular oxygen and deoxymyoglobin containing a weakly coordinated water molecule. The neutral side chain of the distal histidine (His64) inhibits autooxidation by hydrogen bonding to bound oxygen, preventing both HO2 dissociation and the oxidative bimolecular reaction with deoxymyoglobin. Replacement of His64 by amino acids incapable of hydrogen bonding to the bound ligand markedly increases the rate of autooxidation and causes the superoxide mechanism to predominate. Increasing the polarity of the distal pocket by substitution of Val68 with Ser and Thr accelerates autooxidation, presumably by facilitating protonation of the Fe(II).O2 complex. Increasing the net anionic charge at the protein surface in the vicinity of the heme group also enhances the rate of autooxidation. Decreasing the volume of the distal pocket by replacing small amino acids with larger aliphatic or aromatic residues at positions 68 (E11) and 29 (B10) inhibits autooxidation markedly by decreasing the accessibility of the iron atom to solvent water molecules.