In the course of studies on the oxidation of reduced pyridine nucleotides by the accessory glands, it was found (136a) that the rat seminal vesicle contained a highly active, soluble enzyme that catalyzed the oxidation of reduced ribosyl nicotinamide (NPH), but not of TPNH or DPNH, by menadione (vitamin K3). NRH is readily formed in the accessory glands from DPNH and TPNH by the successive actions of nucleotide pyrophosphatase and 5'-nucleotidase.The NRH-oxidizing enzyme was found to be widely distributed among animal tissues. It was purified more than 100-fold from the soluble cytoplasmic sap of rat kidney, and more than 400-fold from beef kidney. The purified enzyme also catalyzed the menadione-dependent oxidation of a number of N'-alkyl derivatives of dihydronico-tinamide, but it was completely inert toward TPNH, DPNH, the 3-acetylpyridine analogue of DPNH, and reduced nicotinamide mononucleotide. The specificity of the NRH-oxidizing enzyme toward hydrogen or electron acceptors was narrow. In addition to menadione, vitamins K2(S) and K2(10) and coenzyme Q2 functioned as acceptors; but vitamin K„ coenzyme Q10, and many other quinones and also dyes were inactive in this respect. The enzyme did not catalyze the reduction of cytochrome c by NRH unless catalytic amounts of menadione were added; in the latter case, the dihydromenadione formed by the action of the enzyme reduced cytochrome c non-enzymatically. The activity of the purified NRH-oxidizing enzyme was unaffected by the addition of FAD or FMN. However, if the enzyme was treated with acid in ammonium sulfate, more than 80 per cent of the activity was lost, and the treated enzyme could be reactivated by FMN or FAD (but not by riboflavin). The purified NRH-oxidizing enzyme contains FAD as the sole flavin, and does not seem to be identical with any known flavoprotein. Unlike some other flavoproteins that catalyze the oxidation of DPNH and TPNH by menadione (e.g., Marki, F., and Martius, C. Biochem. Z., 331: m, i960), the NRH-oxidizing enzyme is very insensitive to Dicumarol. Estradiol-17/3 and a number of other hydrophobic phenols inhibited strongly the enzymatic oxidation of NRH and N'-alkyl-dihydronicotinamides, but no parallelism between inhibitory action and the estrogenic activity of various phenols was noted. Certain poly-cyclic aromatic hydrocarbons were found to be extremely potent inhibitors of the NRH-oxidizing enzyme (137a). During these studies, it was observed that many carcinogenic aromatic hydrocarbons form red-colored charge transfer complexes with various vitamin K's and other naphthoquinones (137a). Experiments with a variety of electron acceptors fully corroborated the recent findings of A. Szent-Gyorgyi, I. Isenberg, and S. L. Baird (Proc. Nat. Acad. Sc., U.S., 46:1444, i960) that ability to act as strong π-electron donors in charge transfer reactions is a necessary (although not sufficient) condition for polycyclic aromatic hydrocarbons to exhibit carcinogenic properties.