AbstractHomogenates of all rat tissues examined, except brain, catalyze reduction ofN,N‐dimethyl‐p‐aminoazobenzeneN‐oxide (DMABN‐oxide) toN,N‐dimethyl‐p‐aminoazobenzene by NADPH. Liver is the most active, and about one third of the homogenate activity of this tissue is recovered in the cytosol fraction. The purified cytosol enzyme has the properties of a tetrameric protein (Mr 370,000) consisting of identical subunits free from chromophores that absorb in the visible spectrum and from metals or other detectable prosthetic groups. The purified reductase is also free from NADPH oxidase and from cytochromecor azo reductase activities. The enzyme is quite specific for NADPH as reductant and DMABN‐oxide as the electron acceptor. Reduction of otherN,N‐dimethyl‐arylamine or alkylamine oxides as well asN‐methylheterocyclicamine oxides could not be detected. Analysis of kinetic data indicate that, at saturating concentrations of the other substrate, 21 μM NADPH and 700 μM DMABN‐oxide are required for half maximal velocity. At infinite concentrations of both substrates the tu