The nitrate reductase of Escherichia coli K-12 was localized in a particulate fraction of the cell and it sedimented as if it were bound to a large substructure that is subject to fragmentation during cell disruption procedures. Soluble enzyme, exhibiting a homogenous profile in sucrose gradients, was released from this fraction by an alkaline-heat treatment. Less than 1.5% of total active nitrate reductase apparently occurred in this soluble form during the course of formation of the particulate enzyme. Enzyme synthesis was repressed by aeration in the presence or absence of nitrate. Under anaerobic conditions, nitrate reductase was synthesized at a rate that could be increased 20-fold by the addition of nitrate. When enzyme synthesis was initiated by induction with nitrate or anaerobiosis, biphasic kinetics were obtained. We interpreted the results as evidence for the existence of a redox-sensitive repressor which mediates nitrate reductase regulation.