Time-resolved study of fluorescence decay of the tryptophan residue in bovine cytochrome c oxidase in phospholipid vesicles is reported for the first time. The effect of the redox state of the protein on its conformation has been investigated using time-resolved decay of tryptophan fluorescence in the oxidised and reduced protein. The fluorescence decay was best fitted using a discrete three exponential model. Amplitude distribution of lifetimes also showed three distinct regions in the analysis of decay profiles by the maximum entropy method (MEM). Results of the time resolved studies showed that the amplitudes as well as the lifetimes of the tryptophan fluorescence remain the same for the oxidised and the reduced states of cytochrome c oxidase, indicating that the environment around tryptophan residues remains more or less unaltered on reduction of the protein. The results suggest that there is no global conformational change in the protein on electron transfer and support the possibility of the existence of local fluctuations in the protein during the redox cycle.