Cytochrome oxidase (EC 1.9.3.1; ferrocytochrome c:oxygen oxidoreductase) was studied during steady-state by optical and e.p.r. methods. Starting with either the 'resting' or the 'pulsed' enzyme, oxidase, cytochrome c, ascorbate and O2 were mixed and the reaction monitored optically. Tetramethylphenylenediamine was used as mediator to poise the steady-state to the desired reduction level. After mixing, the reaction was quenched by the used of rapid-freeze techniques. The e.p.r. spectra of samples captured at increasing tetramethylphenylenediamine concentrations (i.e. higher electron flux) show decreasing g = 2 (Cu A) and g = 3 (cytochrome a) signals. No Cu B or g = 6 signals (high-spin cytochrome a3) could be found during the reaction. Also, the signal with peaks at g = 1.69, 1.78 and 5 as well as the g = 12 signal was hardly detectable at higher turnover rates. The only new signal appearing during turnover is a radical signal, which is discussed in terms of a protein radical. Finally, a scheme is presented, proposing a catalytic cycle for cytochrome oxidase with respect to the O2 binding Cu B-cytochrome a3 unit.