Abstract: The cells of Anabaena azotica reduce the colourless TTC and NBT into red or blue water-insoluble formazans respectively. Heterocysts reduce TTC faster than the vegetative cells. NBT has revealed the strongest reducing power in proheterocysts and vegetative cells next to heterocysts. There is none of reduced NBT in heterocysts.Both red formazan deposition in heterocysts and blue-formazan formation in vegetative cells inhibit nitrogenase activity. The NBT formazan more inhibits the nitrogenase activity than TTC formazan, because the potential redox of NBT is lower than TTC.Both TTC and NBT conspicuously inhibit hydrogen evolution, as a result of catalysis of nitrogenase in intact cells of Anabaena azotica. It is considered that tetrazolium intercepts the electrons from the system catalyzed by nitrogenase.There is controversy about the relationship between tetrazolium reduction and uptake hydrogenase activity in nitrogen fixing organisms (Cyanobacteria and Rhizobia). Some scientists consider that the reduction of TTC in agar medium is used to dectect strains of free living Rhizobia capable of oxidizing H2, that is use of TTC as a method for detecting colonies with uptake hydrogenase is advantageous. On the contrary, we discover that after reducing TTC and NBT by Anabaena azotica there is no effect on H2-uptake capacity. So we suppose that the reduction of tetrazolium does not directly relate to uptake hydrogenase.