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A review, with 63 refs. Vanadate stimulates oxidn. of NADH and NADPH nonenzymically and the rate of the reaction is enhanced by enzymes in plasma and microsomal membranes with NADH, and by xanthine oxidase with both NADH and NADPH. This $H_2O_2$-generating reaction, with a stoichiometry of $NADH-O_2-H_2O_2$ of 1:1:1, is unusual in its inhibition by SOD which made its acceptance initially difficult. The first equiv. of NADH being used for a SOD-insensitive redn. of vanadate to a blue compd. in microsomes, caution is suggested in interpretation of SOD effects. The activity is high with polymeric vanadate. The low activity with metavanadate increases on acidification/polymn., chem. redn. to vanadyl and presence of $H_2O_2$, or converting into oxo-peroxo vanadate by treatment with $H_2O_2$. A common structural feature of O:V-O-V:O with IR band of 985 $cm^{-1}$ is noticed in all active vanadate forms. Mechanisms not involving superoxide need to be considered now in view of doubts on its formation in this reaction. Vanadate participates in several redox activities such as NADH oxidn., nitrogenase, lipid peroxidn. and bromoperoxidase, with H-abstraction being the core reaction.