The kinetics of the reaction of cytochrome c with solubilized mammalian cytochrome c oxidase (ferrocytochrome c:oxygen oxidoreductase, EC 1.9.3.1) has been studied by a stopped-flow technique under two different experimental situations: (i) the completely oxidized enzyme (resting oxidase as obtained from the preparation) was mixed with reduced cytochrome c, and (ii) the completely reduced enzyme in the presence of reduced cytochrome c was exposed to a "pulse" of O2 (pulsed oxidase). Both sets of experiments were performed with either "limiting" or "excess" O2 (relative to oxidase), in the presence or absence of CO. Both the pre-steady-state events and the steady-state kinetics of cytochrome oxidase are found to be different in the two cases. This shows that the product of the reaction of fully reduced oxidase with O2 (pulsed oxidase) is functionally different from the oxidase as prepared (resting oxidase). These differences are interpreted with the assumption of a different rate of intramolecular electron transfer in the pulsed and resting oxidases. Implications of these experimental findings are discussed in the general framework of a tentative model for the catalytic cycle of the oxidase.