The respiration of nitrous oxide to dinitrogen was inhibited by nitric oxide in Pseudomonas perfectomarina and in other denitrifying bacteria. The sensitivity of N2O respiration towards NO was correlated with the growth mode under which the denitrifying system was expressed. N2O respiration was more sensitive to NO in N2O-grown cells than in nitrate-grown cells, exhibiting apparent Ki values of 0.35 ± 0.1 kPa and 5.5 ± 0.6 kPa, respectively. In both cell types no competitive relation between NO and N2O was observed. Cells grown under N2O had only weak NO reductase activity, whereas cells grown with nitrate showed an at least 3-fold higher rate of NO reduction under 10 kPa NO. Oxygen limitation during expression of the denitrifying system also led to a differential expression of individual reaction steps. This, together with inhibition of N2O respiration by NO, may cause the large variation of the products of denitrification formed under oxygen-limited conditions in several representative denitrifying Pseudomonads. NO reacted with the purple form of purified N2O reductase, converting its electronic absorption spectrum to the pink form concomitant with a loss of most of the catalytic activity. Removal of NO and subsequent air oxidation of the enzyme largely restored the original spectrum, but not the activity.