Abstract: A novel method for initiating intramolecular electron transfer in cytochrome c oxidase is reported. The method is based upon photoreduction of cytochrome c labeled with thiouredopyrene-3,6, 8-trisulfonate in complex with cytochrome oxidase. The thiouredopyrene-3,6,8-trisulfonate-labeled cytochrome c was prepared by incubating the thiol reactive form of the dye with yeast iso-1-cytochrome c, containing a single cysteine residue. Laser pulse excitation of a stoichiometrical complex between thiouredopyrene-3,6,8-trisulfonate-cytochrome c and bovine heart cytochrome oxidase at low ionic strength resulted in the reduction of cytochrome c by the excited form of thiouredopyrene-3,6, 8-trisulfonate and subsequent intramolecular electron transfer from the reduced cytochrome c to cytochrome oxidase. The maximum efficiency by a single laser pulse resulted in the reduction of approximately 17% of cytochrome a, and was achieved only at a 1: 1 ratio of cytochrome c to cytochrome oxidase. At higher cytochrome c to cytochrome oxidase ratios the heme a reduction was strongly suppressed.