1. 1. The effect of nitrate on enzyme levels in batch cultures of Escherichia coli grown aerobically or anaerobically has been studied. 2. 2. In cells grown anaerobically in the presence of nitrate, aconitase (EC 4.2.1.3), fumarase (EC 4.2.1.2) and fumarate reductase activities are extremely low; malate and isocitrate dehydrogenase (EC 1.1.1.37 and EC 1.1.1.42) show typically anaerobic activities. Hydrogenase and hydrogenlyase are absent in these cells whilst nitrate reductase and methylene blue linked formate dehydrogenase (EC 1.2.2.1) activities are high. 3. 3. Experiments with a chemostat, in which the nitrate concentration was varied systematically during anaerobic growth, showed that low concentrations of nitrate stimulate biosynthesis of large amounts of cytochrome c 552, but higher concentrations are required for maximum synthesis of cytochrome b 1. 4. 4. Nitrate is reduced more rapidly by formate than by glucose in washed cells which have high levels of formate dehydrogenase. 5. 5. The relevance of these results to proposed pathways of nitrate reduction and the regulation of enzyme activities is discussed. It is suggested that the redox potential of the medium rather than the presence of a specific terminal electron acceptor, regulates enzymes involved in energy metabolism and electron-transport pathways.