Abstract Rates of oxygen utilization by Pseudomonas putida respiratory particles were measured using the electron donors, reduced nicotinamide adenine dinucleotide (NADH) and succinate, and the oxidation-reduction dyes, 2,6-dichlorophenolindophenol and N,N,N′,N′-tetramethyl- p-phenylenediamine. The maximal rates produced by NADH and succinate were similar for particles from either log- or stationary-phase cells, but rates measured using the dyes were much higher in stationary-phase particles. Cyanide and azide were very effective inhibitors of dye oxidation in both cases, but they produced only partial inhibition of NADH and succinate oxidation in log-phase particles and had no effect in the stationary phase. Spectral examination of the cytochromes at several levels of reduction produced by the various electron donors and inhibitors indicated that most of the cytochromes that were reduced by the dyes lie on a cyanide sensitive pathway of electron transport. These findings support the hypothesis that P. putida produces an electron transport system in the stationary phase which involves branching at the level of the cytochromes. Inhibition of oxygen utilization by CO was nearly complete for all four substrates in logphase particles. Inhibition was also reasonably effective for dye oxidation in the stationary phase, but there was no effect on NADH or succinate oxidation. Photochemical action spectra of the relief of CO inhibition revealed that NADH and succinate oxidation in log-phase particles probably involves cytochrome o. Oxidation of the dyes by either type of particles also appeared to involve cytochrome o, and the possibility of the participation of an a- or d-type cytochrome was also indicated.