Nitrogenase activity of cells of Derxia gummosa (30 h growth in cultures without combined nitrogen) was not inhibited on adding nitrate. However, on adding either azaserine or methionine sulfoximine (MSX) with nitrate to these cells, nitrogenase (C2H2 reduction) was inhibited because nitrite accumulated in the reaction mixtures. Nitrite inhibition of the in vivo C2H2 reduction had a Ki value of 16 μM. Both ammonia and glutamine inhibited N2 fixation (C2H2 reduction) in intact cells and in those treated with toluene. This inhibition by ammonia was relieved by methionine sulfoximine but not by glutamine. Azaserine enhanced the inhibition of nitrogenase produced by either ammonia or glutamine, since these treatments resulted in an accumulation of glutamine.