Intramolecular electron transfer (ET) between the CuA center and heme a in bovine cytochrome c oxidase was investigated by pulse radiolysis. CuA, the initial electron acceptor, was reduced by 1-methyl nicotinamide radicals in a diffusion-controlled reaction, as monitored by absorption changes at 830 nm. After the initial reduction phase, the 830 nm absorption was partially restored, corresponding to reoxidation of the CuA center. Concomitantly, the absorption at 445 nm and 605 nm increased, indicating reduction of heme a. The rate constants for heme a reduction and CuA reoxidation were identical within experimental error and independent of the enzyme concentration. This demonstrates that a fast intramolecular electron equilibration is taking place between CuA and heme a. The rate constants for CuA --> heme a ET and the reverse (heme a --> CuA) process were found to be 13 000 s-1 and 3700 s-1, respectively, at 25 degrees C and pH 7.4. This corresponds to an equilibrium constant of 3.4 under these conditions. Thermodynamic and activation parameters of the ET reactions were determined. The significance of these results, particularly the observed low activation barriers, are discussed within the framework of the known three-dimensional structure, ET pathways and reorganization energies.