The enzymatic complex nitrate reductase from Chlorella fusca is inactivated by simple thiols, like dithioerythritol and mercaptoethanol, and by sulfite. The inactivation affects FNH 2-nitrate reductase but not NADH-diaphorase. In all cases, partial reactivation can be achieved by addition of ferricyanide. The extent of inactivation is increased by the presence of NAD +, but not by NADP +. Nitrate, on the contrary, protects the complex against inactivation. Although the interaction of thiols and sulfite with molybdenum or other possible metal present in the complex is not excluded, it seems probable that the inactivation produced by these reagents involves the reduction of disulfide bridges.