The utilization of NO−3, NO−2 and NH+4 was studied in whole filaments and isolated heterocysts of Anabaena 7120 (ATCC27893). NO−3‐ and NO−2‐uptake were detectable in whole filaments but not in heterocysts, whereas NH+4‐uptake was detectable in both. Activity of NO−3‐reductase was present in cell‐free extracts of whole filaments but not of heterocysts, whereas activities of NO−2‐reductase and glutamine synthetase were present in both. NO−3‐uptake and reductase activities could not be induced in heterocysts even after prolonged incubation in NO−3 medium. It is suggested that NO−3‐metabolism in heterocysts is impaired due to a selective and irreversible loss of NO−3‐uptake and reductase systems resulting in the abolition of competition for molybdenum cofactor (Mo‐Co) and reductant between nitrogenase and NO−3‐reductase, and an increase in glucose 6‐phosphate dehydrogenase and 6‐phosphogluconate dehydrogenase levels.