Abstract The enzyme nitrate reductase is a multimeric protein facilitating the flow of electrons from NADH to nitrate through its prosthetic groups FAD, heme and molybdenum cofactor. It also shows partial activities involving one or two of the prosthetic groups. Some mutants known to be deficient in NADH specific nitrate reductase activity are known to assimilate and grow on nitrate. Some alternate functions of the enzyme such as reductive cleavage of the iron siderophores, dissimilatory release of molecular oxygen and chlorate reduction are also known. Unknown functions of the enzyme, especially in the storage tissues, point to the possibility that nitrate reductase may have roles other than the assimilation of nitrate, at least in some tissues, which may perhaps be linked to different isoforms of the enzyme.