The effect of the nitrogen source on carbohydrate and protein contents and on several enzymatic activities involved in the carbon and nitrogen metabolism was studied in Anabaena variabilis ATCC 29413 cells grown under a constant supply of either N, NO−3 or NH+4 at different concentrations. An enhancement of protein content accompanied by a parallel decrease of carbohydrates was observed with increasing NO−3 or NH+4 concentrations in the medium. In cultures containing 0.1 mM NO−3 or 0.1 mM NH+4 nitrogenase (EC 1.18.6.1) activity was 74 and 66%, respectively, of that found in N2‐grown cells. This activity was still present with 1 mM NO−3 or 1 mM NH+4 in the medium and even with 10 mM NO−3, but it was completely inhibited by 5 mM NH+4. Ferredoxin‐nitrate reductase (EC 1.7.7.2) activity was detected only in NO−3 grown cells and simultaneously with nitrogenase activity. Increasing concentrations of combined nitrogen in the medium, especially NH+4, promoted a concomitant decline of glutamine synthetase (EC 6.3.1.2), NADP+‐isocitrate dehydrogenase (EC 1.1.1.42), and NAD+‐malate dehydrogenase (EC 1.1.1.37) activities, suggesting that these enzymes play an important role in the regulation of carbon‐nitrogen metabolism in cyanobacteria.