The biochemical mechanism underlying vanadate-stimulated NAD(P)H oxidation is controversial. Some reports favor an exclusive role for a superoxide (O-2-mediated radical chain reaction, while others cite data that suggest a contribution from O-2-independent enzymatic pathways. We recently reported that a vanadium(IV) species accumulates over a period of about 30 min in phosphate-buffer mixtures of vanadate, NAD(P)H, and a flavoenzyme such as glutathione reductase, lipoyl dehydrogenase, or ferredoxin-NADP+ oxidoreductase. The concentration of this vanadium(IV) species was found to depend critically on the simultaneous presence of the enzyme and NAD(P)H, but not on superoxide dismutase, or a nitrogen atmosphere. It was thus concluded that the flavoenzyme/NAD(P)H system acts as a vanadate reductase. However, a subsequent report put forth an alternative hypothesis in which the accumulation of this vanadium(IV) species is ascribed to direct reduction of vanadate by NAD(P)H itself, starting when buffer-dissolved molecular O2 and H2O2 have been depleted. We have reexamined our earlier data, and carried out new measurements to evaluate the effect of dissolved oxygen and related factors on the kinetics of vanadium(IV) generation in vanadate/NAD(P)H/flavoenzyme mixtures. The new data support our earlier suggestion that the above-mentioned fiavoenzymes can indeed act as NAD(P)H-dependent vanadate reductases.