The heterocyst formations from the initiation and differentia- tion of vegetative cells of filamentous Anabaena HA 101 were inhibited by the treatment of ammonium nitrate. Besides, those heterocysts, which had already formed along the filaments, could be hastened to degenerate by the ammonium nitrate. The nitrogen fixing (acetylene reducing) and hydrogen evolving activity of nitrogenase localized in heterocysts were closely related with the level of heterocyst formations and heterocyst degenerations. The nitrogenase activity for hydrogen production was decreased while the Anabaena were incubated with a gas mixture containing dinitiogen, which is the substrate of nitrogenase, but the enzyme activity was increased while the Anabaena were grown under a dinitrogen-limited condition. The oxygen was needed for the energy production for the nitrogen fixation and hydrogen evolution mediated by the nitrogenase. Although the Anabaena were treated with a gas mixture containing 20% oxygen, the nitrogenase in heterocysts remained high activities for nitrogen fixation and hydrogen production. The inhibitor, KCN, but not DCMU, suppressed the nitrogenase to reduce acetylene and to evolve hydrogen. This suggests that the energy required for nitrogenase activities was derived from oxidative phosphorylation of respiratory process but was not from the photophosphorylation of photosynthetic process.