Added vanadate ions inhibit purified nitrate reductase from Chlorella vulgaris by reacting with the enzyme in a manner rather similar to that of HCN. Thus vanadate, like HCN, forms an inactive complex with the reduced enzyme, and this inactivated enzyme can be reactivated rapidly by adding ferricyanide. The inactive vanadate enzyme complex is less stable than the inactive HCN complex, and the two can be distinguished by the fact that EDTA causes a partial reactivation of the former, but not of the latter. Vanadate can also cause an increase in HCN formation by intact Chlorella vulgaris cells. When these cells were incubated with vanadate, their nitrate reductase was reversibly inactivated, and all of this inactive enzyme could be shown to be the HCN complex rather than the vanadate complex. When HCN and vanadate are both present, the HCN-inactivated enzyme, being more stable, will be formed in preference to the vanadate-inactivated enzyme.