Anaerobic nitrate respiration is regulated by oxygen at the level of nitrate transport; however, the mechanism of O2 inhibition is unknown. Potentially, oxygen could inhibit directly by causing conformational changes in the porter system or indirectly through diversion of electron flow from the nitrate reductase complex to oxygen reduction. Inhibition due to electron diversion implies that nitrate reduction is required for nitrate transport. In this regard, nitrate uptake and its regulation by oxygen were studied in mutants of Escherichia coli (strain AN386) deficient in cytochrome d (RG98), cytochrome o (RG101), and a mutant deficient in both cytochrome d and cytochrome o (RG99). Respiratory nitrate uptake in RG99 was highly resistant to the effects of oxygen supporting the indirect mechanism of electron diversion in oxygen regulation. Nitrate transport in RG98 and RG101 is highly sensitive to oxygen; these mutants exhibited 81 and 85% inhibition, respectively, which is similar to inhibition in the wild type. These results indicate that during nitrate respiration, O2 inhibits transport by limiting the supply of electrons to the nitrate reductase complex.