In the presence of light, reduced nicotinamide adenine dinucleotide (NADH) and riboflavin formed a complex which was able to reduce certain tetrazolium salts. Neither NADH (10−3 M) nor riboflavin (10−4 M) alone was able to induce tetrazolium reduction in the presence of oxygen, but in a nitrogen atmosphere photoreduction of riboflavin induced reduction of tetrazolium salts. Only electrophilic nitro and thiazolyl substituted tetrazolium salts with more positive redox potentials were reduced by the NADH-riboflavin complex, and only monoformazans were produced from the ditetrazolium salts. The reduction kinetics of these tetrazolium salts are given, and the spectral area capable for induction of electron transfer in the NADH-riboflavin complex is screened. It is concluded that the electron transfer in flavin nucleotide dependent dehydrogenase systems will probably proceed without direct interference with the apoenzyme. This may have practical implications for the histochemistry of tetrazolium reductases especially as regards fixation. The catalytic action of light on tetrazolium reduction should also be taken into consideration when tetrazolium salts are used as electron acceptors in a histochemical reaction.