Nitrate-grown Azotobacter chroococcum ATCC 4412 cells lack the ability to fix N2. Nitrogenase activity developed after the cells were suspended in a combined nitrogen-free medium and was paralleled by a concomitant decrease in nitrate assimilation capacity. In such treated cells exhibiting transitory nitrate assimilation and N2-fixation capacity, nitrate or nitrite caused a short-term inhibitory effect on nitrogenase activity which ceased once the anion was exhausted from the medium. The analog L-methionine-DL-sulfoximine, an inhibitor of glutamine synthetase, prevented inhibition of nitrogenase activity by nitrate or nitrite without affecting the uptake of these antions, which were reduced and stoichiometrically released into the external medium as ammonium. Inhibition of nitrogenase by nitrate (nitrite) did not take place in A. chroococcum MCD1, which is unable to assimilate either. We conclude that the short-term inhibitory effect of nitrate (nitrite) on nitrogenase activity is due to some organic product(s) formed during the assimilation of the ammonium resulting from nitrate (nitrite) reduction.