Sandy loam soil, with added glucose, was incubated anaerobically under N 2 and subjected to repeated 1-h C 2H 2 reduction assays. In the presence of 1% glucose the addition of 50 μg NH 4 + −N/g or of 20 μg NO − 3 N/g (untreated soil contained 1.2 μg NH + 4−N and 7.10 μg NO − 3-N/g) caused at least some suppression of nitrogenase activity. Activity developed when the KCl-extractable soil inorganic nitrogen concentration dropped below 35 μg/g. In the presence of 0.1 or 0.05% glucose the addition of 5 μg NH + 4−N/g caused some suppression of nitrogenase activity. However, activity developed when the soil NH 4 +-N concentration dropped below about 4 μg/g. With 0.1% glucose and 5 μg added NO − 2 N/g, activity did not develop until the soil NO − 2 -N concentration dropped to zero. Added NO − 3 N was rapidly reduced and denitrified to NO − 2- N, N 2O-N and NH + 4 N and furthermore caused some inhibition of CO 2 evolution. The data from NH 4 −-addition experiments are consistent with a nitrogenase repression/ derepression threshold of 4 and 35μg NH + 4-N/g at 0.05 and 1% glucose concentrations, respectively. The data from NO − 2- and NO − 3-addition experiments suggest a combination of repression and toxicity effects in the presence of added NO − 3 N.