In the heterocystous cyanobacterium Anabaena variabilis, a change in nitrogenase activity and concomitant modification of dinitrogenase reductase (the Fe protein of nitrogenase) was induced either by NH4Cl at pH 10 (S. Reich and P. Böger, FEMS Microbiol. Lett. 58:81-86, 1989) or by cessation of C supply resulting from darkness, CO2 limitation, or inhibition of photosystem II activity. Modification induced by both C limitation and NH4Cl was efficiently prevented by anaerobic conditions. Under air, endogenously stored glycogen and added fructose protected against modification triggered by C limitation but not by NH4Cl. With stored glycogen present, dark modification took place after inhibition of respiration by KCN. Reactivation of inactivated nitrogenase and concomitant demodification of dinitrogenase reductase occurred after restoration of diazotrophic growth conditions. In previously C-limited cultures, reactivation was also observed in the dark after addition of fructose (heterotrophic growth) and under anaerobiosis upon reillumination in the presence of a photosynthesis inhibitor. The results indicate that modification of dinitrogenase reductase develops as a result of decreased carbohydrate-supported reductant supply of the heterocysts caused by C limitation or by increased diversion of carbohydrates towards ammonia assimilation. Apparently, a product of N assimilation such as glutamine is not necessary for modification. The increase of oxygen concentration in the heterocysts is a plausible consequence of all treatments causing Fe protein modification.